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9: Genetics - Biology

9: Genetics - Biology


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  • 9.1: Introduction
    In the mid-1800s, an Augustinian friar named Gregor Mendel formalized quantitative observations on heredity in the pea plant. He undertook hybridization experiments that utilized purebred or true-breeding plants with specific qualities over many generations to observe the passage of these traits. Some of these physical traits included: seed shape, flower color, plant height, and pod shape.
  • 9.2: Bitter Taste (Activity)
    Some of our personal preferences arise from the way we were brought up. Culture plays a role in our likes and dislikes. Likewise, our experiences play a role in how we respond to certain stimuli. In our search for nutritive compounds, we have learned to avoid things that don’t taste good. Bitter things have a tendency to be associated with toxic compounds in nature. Hence, something bitter might make us learn to avoid this food item in the future.
  • 9.3: Sex-linked Genes
    For the most part, mammals have gender determined by the presence of the Y chromosome. This chromosome is gene poor and a specific area called sex determining region on Y (SRY) is responsible for the initiation of the male sex determination. The X-chromosome is rich in genes while the Y-chromosome is a gene desert. The presence of an X-chromosome is absolutely necessary to produce a viable life form and the default gender of mammals is traditionally female.
  • 9.4: Probability and Chi-Square Analysis
    Punnett Squares are convenient for predicting the outcome of monohybrid or dihybrid crosses. The expectation of two heterozygous parents is 3:1 in a single trait cross or 9:3:3:1 in a two-trait cross. Performing a three or four trait cross becomes very messy. In these instances, it is better to follow the rules of probability. Probability is the chance that an event will occur expressed as a fraction or percentage.
  • 9.5: Non-Mendelian Genetics
    During Mendel’s time, people believed in a concept of blending inheritance whereby offspring demonstrated intermediate phenotypes between those of the parental generation. This was refuted by Mendel’s pea experiments that illustrated a Law of Dominance. Despite this, non-Mendelian inheritance can be observed in sex-linkage and co-dominance where the expected ratios of phenotypes are not observed clearly.
  • 9.6: Hardy-Weinberg and Population Genetics
    The Hardy-Weinberg principle is a mathematical model used to describe the equilibrium of two alleles in a population in the absence of evolutionary forces. This model was derived independently by G.H. Hardy and Wilhelm Weinberg. It states that the allele and genotype frequencies across a population will remain constant across generations in the absence of evolutionary forces.

Glossary of genetics

This glossary of genetics is a list of definitions of terms and concepts commonly used in the study of genetics and related disciplines in biology, including molecular biology and evolutionary biology. [1] It is intended as introductory material for novices for more specific and technical detail, see the article corresponding to each term. For related terms, see Glossary of evolutionary biology.

Also rendered as three-prime end.

The end of a single strand of DNA or RNA at which the chain of nucleotides terminates at the third carbon atom in the furanose ring of deoxyribose or ribose (i.e. the terminus at which the 3' carbon is not attached to another nucleotide via a phosphodiester bond in vivo, the 3' carbon is often still bonded to a hydroxyl group). By convention, sequences and structures positioned nearer to the 3'-end relative to others are referred to as downstream . Contrast 5'-end .

Also rendered as five-prime cap.

A specially altered nucleotide attached to the 5'-end of some primary RNA transcripts as part of the set of post-transcriptional modifications which convert raw transcripts into mature RNA products. The precise structure of the 5' cap varies widely by organism in eukaryotes, the most basic cap consists of a methylated guanine nucleoside bonded to the triphosphate group that terminates the 5'-end of an RNA sequence. Among other functions, capping helps to regulate the export of mature RNAs from the nucleus , prevent their degradation by exonucleases , and promote translation in the cytoplasm. Mature mRNAs can also be decapped.

Also rendered as five-prime end.

The end of a single strand of DNA or RNA at which the chain of nucleotides terminates at the fifth carbon atom in the furanose ring of deoxyribose or ribose (i.e. the terminus at which the 5' carbon is not attached to another nucleotide via a phosphodiester bond in vivo, the 5' carbon is often still bonded to a phosphate group). By convention, sequences and structures positioned nearer to the 5'-end relative to others are referred to as upstream . Contrast 3'-end .

Abbreviated in shorthand with the letter A .

One of the four main nucleobases present in DNA and RNA . Adenine forms a base pair with thymine in DNA and with uracil in RNA. Any pair of organisms which are related genetically and both affected by the same trait . For example, two cousins who both have blue eyes are an affected relative pair since they are both affected by the allele that codes for blue eyes. One of multiple alternative versions of an individual gene , each of which is a viable DNA sequence occupying a given position, or locus , on a chromosome . For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes. The relative frequency with which a particular allele of a given gene (as opposed to other alleles of the same gene) occurs at a particular locus in the members of a population more specifically, it is the proportion of all chromosomes within a population that carry a particular allele, expressed as a fraction or percentage. Allele frequency is distinct from genotype frequency , although they are related.

Also called a sex chromosome, heterochromosome, or idiochromosome.

Any chromosome that differs from an ordinary autosome in size, form, or behavior and which is responsible for determining the sex of an organism. In humans, the X chromosome and the Y chromosome are sex chromosomes.

Also called differential splicing or simply splicing.

A regulated phenomenon of eukaryotic gene expression in which specific exons or parts of exons from the same primary transcript are variably included within or removed from the final, mature messenger RNA transcript. A class of post-transcriptional modification , alternative splicing allows a single gene to code for multiple protein isoforms and greatly increases the diversity of proteins that can be produced by an individual genome . See also RNA splicing . An organic compound containing amine and carboxyl functional groups, as well as a side chain specific to each individual amino acid. Out of nearly 500 known amino acids, a set of 20 are coded for by the standard genetic code and incorporated in sequence as the building blocks of polypeptides and hence of proteins . The specific sequence of amino acids in the polypeptide chains that form a protein are ultimately responsible for determining the protein's structure and function. The stage of mitosis and meiosis that occurs after metaphase and before telophase , when the replicated chromosomes are segregated and each of the sister chromatids are moved to opposite sides of the cell. The condition of a cell or organism having an abnormal number of one or more specific individual chromosomes (but excluding abnormal numbers of complete sets of chromosomes, which instead is known as euploidy ). A phenomenon by which the symptoms of a genetic disorder become apparent (and often more severe) at an earlier age in affected individuals with each generation that inherits the disorder. A series of three consecutive nucleotides within a transfer RNA which complement the three nucleotides of a codon within an mRNA transcript. During translation , each tRNA recruited to the ribosome contains a single anticodon triplet that pairs with one or more complementary codons from the mRNA sequence, allowing each codon to specify a particular amino acid to be added to the growing peptide chain. Anticodons containing inosine in the first position are capable of pairing with more than one codon due to a phenomenon known as wobble base pairing . The orientation of two strands of a double-stranded nucleic acid (and more generally any pair of biopolymers) which are parallel to each other but with opposite directionality . For example, the two complementary strands of a DNA molecule run side-by-side but in opposite directions, with one strand oriented 5' -to- 3' and the other 3'-to-5'. See template strand . Any chromosome that is not an allosome and hence is not involved in the determination of the sex of an organism. Unlike the sex chromosomes, the autosomes in a diploid cell exist in pairs, with the members of each pair having the same structure, morphology, and genetic loci .

Also called testcrossing.

The breeding of a hybrid organism with one of its parents or an individual genetically similar to one of its parents, often intentionally as a type of selective breeding , with the aim of producing offspring with a genetic identity which is closer to that of the parent. The reproductive event and the resulting progeny are both referred to as a backcross, often abbreviated in genetics shorthand with the symbol BC. A pair of two nucleobases on complementary DNA or RNA strands which are bonded to each other by hydrogen bonds. The ability of consecutive base pairs to stack one upon another contributes to the long-chain double helix structures observed in both double-stranded DNA and double-stranded RNA molecules. A measure of the gene expression level of a gene or genes prior to a perturbation in an experiment, as in a negative control. Baseline expression may also refer to the expected or historical measure of expression for a gene.

Also abbreviated as CAAT box or CAT box.

The conversion of a cell from one tissue-specific cell type to another. This involves dedifferentiation to a pluripotent state an example is the conversion of mouse somatic cells to an undifferentiated embryonic state, which relies on the transcription factors Oct4, Sox2, Myc, and Klf4. [3]

Also called a map unit (m.u.).

A unit for measuring genetic linkage defined as the distance between chromosomal loci for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. Though it is not an actual measure of physical distance, it is used to infer the distance between two loci based on the apparent likelihood of a crossover occurring between them. The part of a chromosome that links a pair of sister chromatids . During mitosis , spindle fibers attach to the centromere via kinetochores. The presence of two or more populations of cells with distinct genotypes in an individual organism, known as a chimera, which has developed from the fusion of cells originating from separate zygotes each population of cells retains its own genome, such that the organism as a whole is a mixture of genetically non-identical issues. Genetic chimerism may be inherited (e.g. by the fusion of multiple embryos during pregnancy) or acquired after birth (e.g. by allogeneic transplantation of cells, tissues, or organs from a genetically non-identical donor) in plants, it can result from grafting or errors in cell division. It is similar to but distinct from mosaicism . One copy of a newly copied chromosome , which is joined to the original chromosome by a centromere . A complex of DNA , RNA , and protein found in eukaryotic cells that is the primary substance comprising chromosomes . Chromatin functions as a means of packaging very long DNA molecules into highly organized and densely compacted shapes, which prevents the strands from becoming tangled, reinforces the DNA during cell division, helps to prevent DNA damage, and plays an important role in regulating gene expression and DNA replication .

Also called crossing over.

The duplication of an entire chromosome , as opposed to a segment of a chromosome or an individual gene . A DNA molecule containing part or all of the genetic material of an organism. Chromosomes may be considered a sort of molecular "package" for carrying DNA within the nucleus of cells and, in most eukaryotes, are composed of long strands of DNA coiled with packaging proteins which bind to and condense the strands to prevent them from becoming an unmanageable tangle. Chromosomes are most easily distinguished and studied in their completely condensed forms, which only occur during cell division . Some simple organisms have only one chromosome made of circular DNA, while most eukaryotes have multiple chromosomes made of linear DNA. A mutation occurring within a cis-regulatory element (such as an operator ) which alters the functioning of a nearby gene or genes on the same strand of DNA. Cis-dominant mutations affect the expression of genes because they occur at sites that control transcription rather than within the genes themselves. Any region of non-coding DNA which regulates the transcription of nearby genes , typically by serving as a binding site for one or more transcription factors . Contrast trans-regulatory element . The branch of genetics based solely on observation of the visible results of reproductive acts, as opposed to that made possible by the modern techniques and methodologies of molecular biology. Contrast molecular genetics . The process of producing, either naturally or artificially, individual organisms or cells which are genetically identical to each other. Clones are the result of all forms of asexual reproduction, and cells that undergo mitosis produce daughter cells that are clones of the parent cell and of each other. Cloning may also refer to biotechnology methods which artificially create copies of organisms or cells, or, in molecular cloning , copies of DNA fragments or other molecules. A type of coregulator that increases the expression of one or more genes by binding to an activator .

Also sense strand, positive (+) sense strand, and nontemplate strand.

The strand of a double-stranded DNA molecule whose nucleotide sequence corresponds directly to that of the RNA transcript produced during transcription (except that thymine bases are substituted with uracil bases in the RNA molecule). Though it is not itself transcribed, the coding strand is by convention the strand used when displaying a DNA sequence because of the direct analogy between its sequence and the codons of the RNA product. Contrast template strand see also sense . A series of three consecutive nucleotides in a coding region of a nucleic acid sequence. Each of these triplets codes for a particular amino acid or stop signal during protein synthesis . DNA and RNA molecules are each written in a language using four "letters" (four different nucleobases ), but the language used to construct proteins includes 20 "letters" (20 different amino acids). Codons provide the key that allows these two languages to be translated into each other. In general, each codon corresponds to a single amino acid (or stop signal), and the full set of codons is called the genetic code . Any non-protein organic compound that is bound to an enzyme. Cofactors are required for the initiation of catalysis. A property of nucleic acid biopolymers whereby two polymeric chains (or "strands") aligned antiparallel to each other will tend to form base pairs consisting of hydrogen bonds between the individual nucleobases comprising each chain, with each of the four types of nucleobase pairing exclusively with one other type of nucleobase e.g. in double-stranded DNA molecules, A pairs only with T and C pairs only with G . Strands that are paired in such a way, and the bases themselves, are said to be complementary. The degree of complementarity between two strands strongly influences the stability of the duplex molecule certain sequences may also be internally complementary, which can result in a single strand binding to itself . Complementarity is fundamental to the mechanisms governing DNA replication , transcription , and DNA repair . DNA that is synthesized from a single-stranded RNA template (typically mRNA or miRNA ) in a reaction catalyzed by the enzyme reverse transcriptase . cDNA is produced both naturally by retroviruses and artificially in certain laboratory techniques, particularly molecular cloning . In bioinformatics, the term may also be used to refer to the sequence of an mRNA transcript expressed as its DNA coding strand counterpart (i.e. with thymine replacing uracil ). See quantitative trait . The controlled, inducible expression of a transgene , either in vitro or in vivo.

Also called a canonical sequence.

A calculated order of the most frequent residues (of either nucleotides or amino acids ) found at each position in a common sequence alignment and obtained by comparing multiple closely related sequence alignments. An interdisciplinary branch of population genetics which applies genetic methods and concepts in an effort to understand the dynamics of genes in populations, principally in order to avoid extinctions and to conserve and restore biodiversity. A nucleic acid or protein sequence that is highly similar or identical across many species or within a genome , indicating that it has remained relatively unchanged through a long period of evolutionary time. The continuous transcription of a gene , as opposed to facultative expression , in which a gene is only transcribed as needed. A gene that is transcribed continuously is called a constitutive gene. A continuous stretch of genomic DNA generated by assembling cloned fragments by means of their overlaps. [2] A phenomenon in which sections of a genome are repeated and the number of repeats varies between individuals in the population, usually as a result of duplication or deletion events that affect entire genes or sections of chromosomes. Copy-number variations play an important role in generating genetic variation within a population. A protein that works together with one or more transcription factors to regulate gene expression . A type of coregulator that reduces (represses) the expression of one or more genes by binding to and activating a repressor .

The breeding of purebred parents belonging to two different breeds, varieties, or populations, often intentionally as a type of selective breeding , with the aim of producing offspring which share traits of both parent lineages or which show heterosis . In animal breeding, the progeny of a cross between breeds of the same species is called a crossbreed, whereas the progeny of a cross between different species is called a hybrid . The branch of genetics that studies how chromosomes influence and relate to cell behavior and function, particularly during mitosis and meiosis .

Abbreviated in shorthand with the letter C .

One of the four main nucleobases present in DNA and RNA . Cytosine forms a base pair with guanine .

Denoted in shorthand with the symbol Δ.

A type of mutation in which one or more bases are removed from a nucleic acid sequence . A polymeric nucleic acid molecule composed of a series of deoxyribonucleotides which incorporate a set of four nucleobases : adenine ( A ), guanine ( G ), cytosine ( C ), and thymine ( T ). DNA is most often found in the form of a " double helix ", which consists of two paired complementary DNA molecules resembling a ladder that has been twisted. The "rungs" of the ladder are made of pairs of nucleobases .

Denoted in shorthand with the somatic number 2n.

(of a cell or organism) Having two homologous copies of each chromosome . Contrast haploid and polyploid . Any quantity used to measure the dissimilarity between the gene expression levels of different genes . [4] The process of compacting very long DNA molecules into densely packed, orderly configurations such as chromosomes , either in vivo or in vitro. A high-throughput technology used to measure expression levels of mRNA transcripts or to detect certain changes in the nucleotide sequence . It consists of an array of thousands of microscopic spots of DNA oligonucleotides , called features, each containing picomoles of a specific DNA sequence. This can be a short section of a gene or other DNA element that is used as a probe to hybridize a cDNA , cRNA or genomic DNA sample (called a target) under high-stringency conditions. Probe-target hybridization is usually detected and quantified by fluorescence-based detection of fluorophore-labeled targets. Any of a class of enzymes that synthesizes DNA molecules from individual deoxyribonucleotides . DNA polymerases are essential for DNA replication and usually work in pairs to create identical copies of the two strands of an original double-stranded molecule. They build long chains of DNA by adding nucleotides one at a time to the 3'-end of a DNA strand, usually relying on the template provided by the complementary strand to copy the nucleotide sequence faithfully. The collection of processes by which a cell identifies and corrects structural damage or mutations in the DNA molecules that encode its genome . The ability of a cell to repair its DNA is vital to the integrity of the genome and the normal functionality of the organism. The process by which a DNA molecule copies itself, producing two identical copies of one original DNA molecule. The process of determining, by any of a variety of different methods and technologies, the order of the bases in the long chain of nucleotides that constitutes a sequence of DNA . A relationship between the alleles of a gene in which one allele produces an effect on phenotype that overpowers or "masks" the contribution of another allele at the same locus the first allele and its associated phenotypic trait are said to be dominant, and the second allele and its associated trait are said to be recessive . Often, the dominant allele codes for a functional protein while its recessive counterpart does not. Dominance is not an inherent property of any allele or phenotype, but simply describes its relationship to one or more other alleles or phenotypes it is possible for one allele to be simultaneously dominant over a second allele, recessive to a third, and codominant to a fourth. In genetics shorthand, dominant alleles are often represented by a single uppercase letter (e.g. "A", in contrast to the recessive "a"). Any mechanism by which organisms neutralize the large difference in gene dosage caused by the presence of differing numbers of sex chromosomes in the different sexes, thereby equalizing the expression of sex-linked genes so that the members of each sex receive the same or similar amounts of the products of such genes. An example is X-inactivation in female mammals. Any DNA molecule that is composed of two antiparallel , complementary nucleotide polymers, or "strands", which are bonded together by hydrogen bonds between the complementary nucleobases . Though it is possible for DNA to exist as a single strand , it is generally more stable and more common in double-stranded form. In most cases, the complementary base pairing causes the twin strands to coil around each other in the shape of a double helix .

Also called repression or suppression.

Any process, natural or artificial, which decreases the level of gene expression of a certain gene . A gene which is observed to be expressed at relatively low levels (such as by detecting lower levels of its mRNA transcripts) in one sample compared to another sample is said to be downregulated. Contrast upregulation . Towards or closer to the 3'-end of a chain of nucleotides . Contrast upstream .

Also called an expression construct.

A type of vector , usually a plasmid or viral vector, designed specifically for the expression of a transgene insert in a target cell, rather than for some other purpose such as cloning . For a given genotype associated with a variable non-binary phenotype , the proportion of individuals with that genotype who show or express the phenotype to a specified extent, usually given as a percentage. Because of the many complex interactions that govern gene expression , the same allele may produce a wide variety of possible phenotypes of differing qualities or degrees in different individuals in such cases, both the phenotype and genotype may be said to show variable expressivity. Expressivity attempts to quantify the range of possible levels of phenotypic variation in a population of individuals expressing the phenotype of interest. Compare penetrance .

Also called extranuclear DNA or cytoplasmic DNA.

Any DNA that is not found in chromosomes or in the nucleus of a cell and hence is not genomic DNA . This may include the DNA contained in plasmids or organelles such as mitochondria or chloroplasts, or, in the broadest sense, DNA introduced by viral infection. Extrachromosomal DNA usually shows significant structural differences from nuclear DNA in the same organism.

Formerly known by the abbreviation MGED.

An organization that works with others "to develop standards for biological research data quality, annotation and exchange" as well as software tools that facilitate their use. [5]

Also Giemsa banding or G-banding.

A technique used in cytogenetics to produce a visible karyotype by staining the condensed chromosomes with Giemsa stain. The staining produces consistent and identifiable patterns of dark and light "bands" in regions of chromatin , which allows specific chromosomes to be easily distinguished. Any segment or set of segments of a nucleic acid molecule that contains the information necessary to produce a functional RNA transcript in a controlled manner. In living organisms, genes are often considered the fundamental units of heredity and are typically encoded in DNA . A particular gene can have multiple different versions, or alleles , and a single gene can result in a gene product that influences many different phenotypes . The number of copies of a particular gene present in a genome . Gene dosage directly influences the amount of gene product a cell is able to express, though a variety of controls have evolved which tightly regulate gene expression . Changes in gene dosage caused by mutations include copy-number variations .

Also called gene amplification.

A type of mutation defined as any duplication of a region of DNA that contains a gene . Compare chromosomal duplication . The process by which the information encoded in a gene is converted into a form useful for the cell. The first step is transcription , which produces a messenger RNA molecule complementary to the DNA molecule in which the gene is encoded. For protein-coding genes, the second step is translation , in which the messenger RNA is read by the ribosome to produce a protein . A database for gene expression managed by the National Center for Biotechnology Information. These high-throughput functional genomics data are derived from experimental data from chips and next-generation sequencing. [6] [7] Any of a variety of methods used to precisely identify the location of a particular gene within a DNA molecule (such as a chromosome) and/or the physical or linkage distances between it and other genes. The sum of all of the various alleles shared by the members of a single population. Any of the biochemical material resulting from the expression of a gene , most often interpreted as the functional mRNA transcript produced by transcription of the gene or the fully constructed protein produced by translation of the transcript. A measurement of the quantity of a given gene product that is detectable in a cell or tissue is sometimes used to infer how active the corresponding gene is. The broad range of mechanisms used by cells to increase or decrease the production or expression of specific gene products , such as RNA or proteins . Gene regulation increases an organism's versatility and adaptability by allowing its cells to express different gene products when required by changes in its environment. In multicellular organisms, the regulation of gene expression also drives cellular differentiation and morphogenesis in the embryo, enabling the creation of a diverse array of cell types from the same genome . Any mechanism of gene regulation which drastically reduces or completely prevents the expression of a particular gene. Gene silencing may occur naturally during either transcription or translation . Laboratory techniques often exploit natural silencing mechanisms to achieve gene knockdown . A high-throughput technology used to simultaneously inactivate, identify, and report the expression of a target gene in a mammalian genome by introducing an insertional mutation consisting of a promoterless reporter gene and/or a selectable genetic marker flanked by an upstream splice site and a downstream polyadenylated termination sequence. The co-occurrence within a population of one or more alleles or genotypes with a particular phenotypic trait more often than might be expected by chance alone such statistical correlation may be used to infer that the alleles or genotypes are responsible for producing the given phenotype. A set of rules by which information encoded within nucleic acids is translated into proteins by living cells. These rules define how sequences of nucleotide triplets called codons specify which amino acid will be added next during protein synthesis . The vast majority of living organisms use the same genetic code (sometimes referred to as the "standard" genetic code ) but variant codes do exist. The process of advising individuals or families who are affected by or at risk of developing genetic disorders in order to help them understand and adapt to the physiological, psychological, and familial implications of genetic contributions to disease. Genetic counseling integrates genetic testing , genetic genealogy , and genetic epidemiology . [8] A measure of the genetic divergence between species, populations within a species, or individuals, used especially in phylogenetics to express either the time elapsed since the existence of a common ancestor or the degree of differentiation in the DNA sequences comprising the genomes of each population or individual.

Sometimes used interchangeably with genetic variation .

The total number of genetic traits or characteristics in the genetic make-up of a population, species, or other group of organisms. It is often used as a measure of the adaptability of a group to changing environments. Genetic diversity is similar to, though distinct from, genetic variability .

Also called allelic drift or the Sewall Wright effect.

A change in the frequency with which an existing allele occurs in a population due to random variation in the distribution of alleles from one generation to the next. It is often interpreted as the role that random chance plays in determining whether a given allele becomes more or less common with each generation, regardless of the influence of natural selection. Genetic drift may cause certain alleles, even otherwise advantageous ones, to disappear completely from the gene pool , thereby reducing genetic variation , or it may cause initially rare alleles, even neutral or deleterious ones, to become much more frequent or even fixed .

Also called genetic modification or genetic manipulation.

The direct, deliberate manipulation of an organism's genetic material using any of a variety of biotechnology methods, including the insertion or removal of genes , the transfer of genes within and between species, the mutation of existing sequences, and the construction of novel sequences using artificial gene synthesis . Genetic engineering encompasses a broad set of technologies by which the genetic composition of individual cells, tissues, or entire organisms may be altered for various purposes, commonly in order to study the functions and expression of individual genes, to produce hormones, vaccines, and other drugs, and to create genetically modified organisms for use in research and agriculture. The use of genealogical DNA testing in combination with traditional genealogical methods to infer the level and type of genetic relationships between individuals, find ancestors, and construct family trees, genograms, or other genealogical charts.

Also called genetic draft or the hitchhiking effect.

A type of linked selection by which the positive selection of an allele undergoing a selective sweep causes alleles for different genes at nearby loci to change frequency as well, allowing them to "hitchhike" to fixation along with the positively selected allele. If selection at the first locus is strong enough, neutral or even slightly deleterious alleles within the same linkage group may undergo the same positive selection because the physical distance between the nearby loci is small enough that a recombination event is unlikely to occur between them. Genetic hitchhiking is often considered the opposite of background selection. A specific, easily identifiable, and usually highly polymorphic gene or other DNA sequence with a known location on a chromosome that can be used to identify the individual or species possessing it. Any reassortment or exchange of genetic material within an individual organism or between individuals of the same or different species, especially that which creates genetic variation . In the broadest sense, the term encompasses a diverse class of naturally occurring mechanisms by which nucleic acid sequences are copied or physically transferred into different genetic environments, including homologous recombination during meiosis or mitosis or as a normal part of DNA repair horizontal gene transfer events such as bacterial conjugation , viral transduction , or transformation or errors in DNA replication or cell division. Artificial recombination is central to many genetic engineering techniques which produce recombinant DNA . A graph that represents the regulatory complexity of gene expression . The vertices (nodes) are represented by various regulatory elements and gene products while the edges (links) are represented by their interactions. These network structures also represent functional relationships by approximating the rate at which genes are transcribed .

Also called DNA testing or genetic screening.

A broad class of various procedures used to identify features of an individual's particular chromosomes, genes, or proteins in order to determine parentage or ancestry, diagnose vulnerabilities to heritable diseases, or detect mutant alleles associated with increased risks of developing genetic disorders . Genetic testing is widely used in human medicine, agriculture, and biological research.

Sometimes used interchangeably with genetic variation .

The formation or the presence of individuals differing in genotype within a population or other group of organisms, as opposed to individuals with environmentally induced differences, which cause only temporary, non-heritable changes in phenotype . Barring other limitations, a population with high genetic variability has a greater potential for successful adaptation to changing environmental conditions than a population with low genetic variability. Genetic variability is similar to, though distinct from, genetic diversity .

Sometimes used interchangeably with genetic diversity and genetic variability .

The genetic differences both within and between populations, species, or other groups of organisms. It is often visualized as the variety of different alleles in the gene pools of different populations. Any organism whose genetic material has been altered using genetic engineering techniques, particularly in a way that does not occur naturally by mating or by natural genetic recombination . The field of biology that studies genes , genetic variation , and heredity in living organisms. The entire complement of genetic material contained within the chromosomes of an organism, organelle, or virus. The term is also used to refer to the collective set of genetic loci shared by every member of a population or species, regardless of the different alleles that may be present at these loci in different individuals. The total amount of DNA contained within one copy of a genome , typically measured by mass (in picograms or daltons) or by the total number of base pairs (in kilobases or megabases ). For diploid organisms, genome size is often used interchangeably with C-value .

Also called chromosomal DNA.

The DNA contained in chromosomes , as opposed to the extrachromosomal DNA contained in separate structures such as plasmids or organelles such as mitochondria or chloroplasts. An epigenetic phenomenon that causes genes to be expressed in a manner dependent upon the particular parent from which the gene was inherited. It occurs when epigenetic marks such as DNA or histone methylation are established or "imprinted" in the germ cells of a parent organism and subsequently maintained through cell divisions in the somatic cells of the organism's progeny as a result, a gene in the progeny that was inherited from the father may be expressed differently than another copy of the same gene that was inherited from the mother. An interdisciplinary field that studies the structure, function, evolution, mapping, and editing of entire genomes , as opposed to individual genes . The ability of certain chemical agents to cause damage to genetic material within a living cell (e.g. through single- or double-stranded breaks, crosslinking , or point mutations ), which may or may not result in a permanent mutation . Though all mutagens are genotoxic, not all genotoxic compounds are mutagenic. The entire complement of alleles present in a particular individual's genome , which gives rise to the individual's phenotype . The process of determining differences in the genotype of an individual by examining the DNA sequences in the individual's genome using bioassays and comparing them to another individual's sequences or a reference sequence. Any biological cell that gives rise to the gametes of an organism that reproduces sexually. Germ cells are the vessels for the genetic material which will ultimately be passed on to the organism's descendants and are usually distinguished from somatic cells , which are entirely separate from the germ line . 1. In multicellular organisms, the population of cells which are capable of passing on their genetic material to the organism's progeny and are therefore (at least theoretically) distinct from somatic cells . The cells of the germ line are called germ cells . 2. The lineage of germ cells, spanning many generations, that contains the genetic material which has been passed on to an individual from its ancestors.

Abbreviated in shorthand with the letter G .

One of the four main nucleobases present in DNA and RNA . Guanine forms a base pair with cytosine .

Also abbreviated GC-content.

The proportion of nitrogenous bases in a nucleic acid that are either guanine ( G ) or cytosine ( C ), typically expressed as a percentage. DNA and RNA molecules with higher GC-content are generally more thermostable than those with lower GC-content due to molecular interactions that occur during base stacking. [9]

Denoted in shorthand with the somatic number n.

(of a cell or organism) Having one copy of each chromosome , with each copy not being part of a pair. Contrast diploid and polyploid . A set of alleles in an individual organism that were inherited together from a single parent. In a diploid organism, having just one allele at a given genetic locus (where there would ordinarily be two). Hemizygosity may be observed when only one copy of a chromosome is present in a normally diploid cell or organism, or when a segment of a chromosome containing one copy of an allele is deleted , or when a gene is located on a sex chromosome in the heterogametic sex (in which the sex chromosomes do not exist in matching pairs) for example, in human males with normal chromosomes, almost all X-linked genes are said to be hemizygous because there is only one X chromosome and few of the same genes exist on the Y chromosome .

Also called inheritance.

The passing on of phenotypic traits from parents to their offspring, either through sexual or asexual reproduction. Offspring cells or organisms are said to inherit the genetic information of their parents. 1. The ability to be inherited . 2. A statistic used in quantitative genetics that estimates the proportion of variation within a given phenotypic trait that is due to genetic variation between individuals in a particular population. Heritability is estimated by comparing the individual phenotypes of closely related individuals in the population. See allosome . The expression of a foreign gene or any other DNA sequence within a host organism which does not naturally contain the same gene. Insertion of foreign transgenes into heterologous hosts using recombinant vectors is a common biotechnology method for studying gene structure and function.

Also called hybrid vigor and outbreeding enhancement.

In a diploid organism, having two different alleles at a given genetic locus . In genetics shorthand, heterozygous genotypes are represented by a pair of non-matching letters or symbols, often an uppercase letter (indicating a dominant allele) and a lowercase letter (indicating a recessive allele), such as "Aa" or "Bb". Contrast homozygous . Any of a class of highly alkaline proteins responsible for packaging nuclear DNA into structural units called nucleosomes in eukaryotic cells. Histones are the chief protein components of chromatin , where they associate into complexes which act as "spools" around which the linear DNA molecule winds. They play a major role in gene regulation and expression .

Also called homologs.

A set of two matching chromosomes , one maternal and one paternal, which pair up with each other inside the nucleus during meiosis . They have the same genes at the same loci , but may have different alleles . A type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical ("homologous") molecules of DNA , especially that which occurs between homologous chromosomes . The term may refer to the recombination that occurs as a part of any of a number of distinct cellular processes, most commonly DNA repair or chromosomal crossover during meiosis in eukaryotes and horizontal gene transfer in prokaryotes. Contrast nonhomologous recombination . In a diploid organism, having two identical alleles at a given genetic locus . In genetics shorthand, homozygous genotypes are represented by a pair of matching letters or symbols, such as "AA" or "aa". Contrast heterozygous . Any constitutive gene that is transcribed at a relatively constant level across many or all known conditions. Such a gene's products typically serve functions critical to the maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions. The offspring that results from combining the qualities of two organisms of different genera, species, breeds, or varieties through sexual reproduction. Hybrids may occur naturally or artificially, as during selective breeding of domesticated animals and plants. Reproductive barriers typically prevent hybridization between distantly related organisms, or at least ensure that hybrid offspring are sterile, but fertile hybrids may result in speciation. 1. The process by which a hybrid organism is produced from two organisms of different genera, species, breeds, or varieties. 2. The process by which a single-stranded DNA or RNA preparation is added to an array surface, in solution, and potentially anneals to the complementary probe . Note that with respect to a gene expression assay, hybridization refers to a step in the experimental paradigm, while in molecular biology or genetics , the term refers to the chemical process.

Also called introgressive hybridization.

The movement of a gene from the gene pool of one population or species into that of another population by the repeated backcrossing of hybrids of the two populations with one of the parent populations. Introgression is a ubiquitous and important source of genetic variation in natural populations, but may also be practiced intentionally in the cultivation of domesticated plants and animals. Any nucleotide sequence within a gene that is removed by RNA splicing during post-transcriptional modification of the mRNA primary transcript and is therefore absent from the final mature mRNA. The term refers to both the sequence as it exists within a DNA molecule and to the corresponding sequence in RNA transcripts. Contrast exon . A type of abnormal chromosome in which the arms of the chromosome are mirror images of each other. Isochromosome formation is equivalent to simultaneous duplication and deletion events such that two copies of either the long arm or the short arm comprise the resulting chromosome.


Lecture 9: Human Genetics

Download the video from iTunes U or the Internet Archive.

Topics covered: Human Genetics

Instructors: Prof. Eric Lander

Lecture 10: Molecular Biolo.

Lecture 11: Molecular Biolo.

Lecture 12: Molecular Biolo.

Lecture 13: Gene Regulation

Lecture 14: Protein Localiz.

Lecture 15: Recombinant DNA 1

Lecture 16: Recombinant DNA 2

Lecture 17: Recombinant DNA 3

Lecture 18: Recombinant DNA 4

Lecture 19: Cell Cycle/Sign.

Lecture 26: Nervous System 1

Lecture 27: Nervous System 2

Lecture 28: Nervous System 3

Lecture 29: Stem Cells/Clon.

Lecture 30: Stem Cells/Clon.

Lecture 31: Molecular Medic.

Lecture 32: Molecular Evolu.

Lecture 33: Molecular Medic.

Lecture 34: Human Polymorph.

Lecture 35: Human Polymorph.

I want to go back a second to the end of last time because in the closing moments there, we, or at least I, got a little bit lost, and where the plusses and minuses were at a certain table.

And, I want to go back and make sure we've got that straight.

We were talking about a situation where we were trying to use genetics, and the phenotypes that might be observed in mutants to try to understand the biochemical pathway because we're beginning to try to unite the geneticist's point of view who looks only at mutants, and the biochemist's point of view who looks at pathways and proteins.

And, I had hypothesized that there was some biochemists who had thought up a possible pathway for the synthesis of arginine that involved some precursor, alpha, beta, gamma, where alpha is turned into beta beta is turned into gamma and gamma is used to turn into arginine. And, hypothetically, there would be some enzymes: enzyme A that converts alpha, enzyme B that converts beta, and enzyme C that converts gamma.

And, we were just thinking about, what would the phenotypes look like of different arginine auxotrophs that had blocks at different stages in the pathway. If I had an arginine auxotroph that had a block here because let's say a mutation in a gene affecting this enzyme, or at a block here at a mutation affecting, say, the gene that encodes enzyme C, how would I be able to tell very simply that they were in different genes? Last time, we found that we could tell they were in different genes by doing a cross between a mutant that had the first mutation, and a mutant that had the second mutation, and looking at the double heterozygote, right? And, if in the double heterozygote you had a wild type or a normal phenotype, then they had to be in different genes, OK? Remember that?

That was called a test of complementation.

That was how we were able to sort out which mutations were in the same gene, and which mutations were in different genes.

Now we can go a step further. When we've established that they're in different genes, we can try to begin to think, how do these genes relate to a biochemical pathway?

I wanted to begin to introduce, because it'll be relevant for today, this notion: so, suppose I had a mutation that affected enzyme A so that this enzymatic step couldn't be carried out.

Such a mutant, when I just try to grow it on minimal medium won't be able to grow. If I give it the substrate alpha, it doesn't do it any good because it hasn't got the enzyme to convert alpha. So, given alpha, it won't grow. But if I give it beta, what will happen? It can grow because I've bypassed the defect. What about if I give it gamma? Arginine?

Now, if instead the mutation were affecting enzymatic step here, then if I give it on minimal or medium but it can grow on gamma. What about this last line?

If I have a mutation and the last enzymatic step, minimal medium can't grow with alpha, can't grow with beta, can't even grow with gamma. But, it can grow with arginine because I've bypassed that step. So, I get a different phenotype, the inability to grow even on gamma, but I can grow on arginine. Now, here, if I put together those mutants and make a double mutant, a double homozygote, let's say, that's defective in both A and B, which will it look like? Will it be able to grow on minimal medium? Will it be able to grow on alpha?

Will it be able to grow on beta?

Will it be able to grow on gamma and arginine? What about if I have a double mutant in B and C, minus, minus, minus, minus, plus? So this looks the same as that. This looks the same as that.

And so, by looking at different mutant combinations, I can see that the phenotype of B here is what occurs in the double mutant. So, this phenotype is epistatic to this phenotype.

Epistatic means stands upon, OK? So, phenotypes, just like phenotypes can be recessive or dominant, you can also speak about them being epistatic. And epistatic means when you have both of two mutations together at the epistatic then one of them is epistatic to the other, perhaps.

It will, in fact, be the one that is present.

So, this is not so easy to do in many cases because if I take different kinds of mutation affecting wing development, and I put them together in the same fly, I may just get a very messed up wing, and it's very hard to tell that the double mutant has a phenotype that looks like either of the two single mutants.

But sometimes, if they fall very nicely in a pathway where this affects the first step, this affects the second step this affects the third step, this affects the fourth step, then the double mutant will look like one of those, OK? And, that way you can somehow order things in a biochemical pathway. Now, notice, this is all indirect, right? This is what geneticists did in the middle of the 20th century to try to figure out how to connect up mutants to biochemistry.

Actually, that's not true. It's what geneticists still do today because you might think that Well, we don't need to do this anymore, but in fact geneticists constantly are looking at mutants and making connections trying to say, what does this double combination look like? What does that double combination look like, and how does that tell us about the developmental pathway, which cell signals which cell? This turns out to be one of the most powerful ways to figure out what mutations do by saying the combination of two mutations looks like the same as one of them, allowing you to order the mutations in a pathway.

And, there's no general way to grind up a cell and order things in a pathway. Genetics is a very powerful tool for doing that.

Now, there are some ways to grind up cells and order things, but you need both of these techniques to believe stuff.

Anyway, I wanted to go over that, because it is an important concept, the concept of epistasis, the concept of relating mutations to steps and pathways, but what I mostly want to do today is go on now to talk about genetics not in organisms like yeast or fruit flies or even peas, but genetics in humans.

So, what's different about genetics in humans than genetics in yeast?

You can't choose who mates with whom. Well, you can.

I mean, in the days of arranged marriages maybe you couldn't, but you can choose who mates with whom, but only for yourself, right? What you can't do is arrange other crosses in the human population as an experimentalist. Now, your own choice of mating, unfortunately or fortunately perhaps produces too few progeny to be statistically significant. As a parent of three, I think about what it would take to raise a statistically significant number of offspring to draw any conclusions, and I don't think I could do that.

So, you're absolutely right. We can't arrange the matings that we want in the human population. So, that's the big difference.

So, can we do genetics anyway? How do we do genetics even though we can't arrange the matings the way we'd like to? Sorry?

Well, family trees. We have to take the matings as we find them in the human population. You can talk to somebody who might have an interesting phenotype, I don't know, attached earlobes, or very early heart disease, or some unusual color of eyes, and begin to collect a family history on that person.

It's a little bit of a dodgy thing because you might just be relying on that person's recollection. So, if you were really industrious about this, you'd go check out each of their family members and test for yourself whether they have the phenotype. People who do serious human genetic studies often go and do that. They have to go confirm, either by getting hospital records or interviewing the other members of the family, etc. So, this is not as easy as plating out lots of yeasts on a Petri plate.

And then you get pedigrees. And the pedigrees look like this.

Here's a pedigree. Tell me what you make of it.

Now, symbols: squares are males, circles are females by convention, a colored in symbol means the phenotype that we're interested in studying at the moment. So, in any given problem, somebody will tell you, well, we're studying some interesting phenotype. You often have an index case or a proband, meaning the person who comes to clinical attention, and then you chase back in the pedigree and try to reconstruct.

So, suppose I saw a pedigree like this.

What conclusions could I draw? Sorry? Recessive, sex link trait why sex link trait? So, let's see if we can get your model up here. You think that this represents sex-linked inheritance. So, what would the genotype be of this male here? Mutant: I'll use M to denote a mutant carried on the X chromosome, and a Y on the opposite chromosome.

What's the genotype of the female here?

So, it's plus over plus where I'll use plus to denote the gene carried on the normal X chromosome. OK, and then what do you think happened over here? So, mutant over plus, you mate to this male who is plus over plus. Why is that male plus over plus? Oh, right, good point.

It's not plus over plus. It's plus over Y. Why is that male plus over Y as opposed to mutant over Y?

He'd have the mutant phenotype. So, he doesn't have the mutant phenotype so he can infer he's plus over Y. OK, and then what happens here? Mutant over Y this is plus over Y. How did this person get plus over Y? They just the plus for mom, and the daughters, Y from dad, and a plus from mom. That's cool. Now, what about the daughters there? They're plus over plus, or M over plus? Is one, one, and one the other? Well, in textbooks it's always plus over plus and M over plus, but in real life? We don't know, right? So, this could be plus over plus, or M over plus, we don't know, OK? Now, what about on this side of the pedigree here?

What's the genotype here? Plus over Y, OK.

Why not mutant over Y? Because if they got the mutant, it would have to come from the, OK, so here, plus over plus, and then here, everybody is normal because there's no mutant allele segregated.

Yes? Yeah, couldn't there just be recessive? I mean, it's a nice story about the sex link but couldn't it be recessive? So, walk me through it being recessive. M over plus, plus over plus. Wait, wait, wait, hang on. Could this be M over plus, and that person be affected?

It's got to be M over M, right so mutants over mutants but that's possible. Yeah, OK. So, what would this person be? Plus over plus, let's say, come over here. Now, what would this person be? M plus. It has to be M plus because, OK, and what about this person here? M plus, now what about the offspring? So, one of them is M over M, plus over plus, and two M pluses. Does it always work out like that?

[LAUGHTER] No, it doesn't always work out like that at all.

So, I'm just going to write plus over plus here just to say, tough, right? In real life, it doesn't always come out like that.

What about over here? It would have to be plus over plus.

Why not? It doesn't because it could be M over plus and have no effect at offspring by chance, right? But, you were going to say it's plus over plus because in the textbooks it's always plus over plus in pictures like this, right? And then, it all turns out to be pluses and mutants, and pluses and mutants, and all that, right? Well, which picture's right?

Sorry? You don't know. So, that's not good. There's supposed to be answers to these things. Could either be true? Which is more likely? The one on the left? Why? More statistically probable, how come? Because it is. It may not quite suffice as a fully complete scientific answer though.

Yes? Yep. Well, but I have somebody who is affected here. So, given that I've gotten affected person in the family -- yeah, so it is actually, you're right, statistically somewhat less likely that you would have two independent M's entering the same pedigree particularly if M is relatively rare.

If M is quite common, however, suppose M were something was a 20% frequency in the population, then it actually might be quite reasonable that this could happen. So, what would you really want to do to test this? Sorry? Well, if you found any females here maybe you'd be able to conclude that it was autosomal recessive because females never show a sex-linked trait. Is that true?

No, that's not true. Why not? You're right. So, you just have to be homozygous for it on the X. So, having a single female won't, I mean, she's not going to take that as evidence. Get an affected female and demonstrate that all of her male offspring show the trait. Cross her with, wait, wait.

This is a human pedigree guys [LAUGHTER]. Whew! There are issues involved here, right? You could introduce her to a normal guy, [LAUGHTER] but whether you can cross her to a normal guy is not actually allowed. So, you see, these are exactly the issues in making sense out of pedigrees like this.

So, what you have to do is you have to collect a lot of data, and the kinds of characteristics that you look for in a pedigree, but they are statistical characteristics, and notwithstanding -- So, this could be colorblindness or something, but notwithstanding the pictures in the textbook of colorblindness and all that, you really do have to take a look at a number of properties. What are some properties?

One you've already referred to which is there's a predominance in males if it's X-linked. Why is there a predominance in males? Well, there's a predominance in males because if I have an X over Y and I've got a mutation paired on this X chromosome, males only have to get it on one.

Females have to get it on both, and therefore it's statistically more likely that males will get it. So, for example, the frequency of colorblindness amongst males is what? Yeah, it's 8-10%, something like that. I think it's about 8% or so.

And, amongst females, well, if it's 8% to get one, what's the chance you're going to get two?

It's 8% times 8% is a little less than 1% right?

It's 0.64%, OK, in females. So, we'll just go 8% squared. So in males, 8% in females, less than one percent.

So, there is a predominance in males of these sex-linked traits. Other things: affected males do not transmit the trait to the kids, in particular do not transmit it to their sons, right, because they are always sending the Y chromosomes to their songs. Carrier females transmit to half of their sons, and affected females transmit to all of their sons. And, the trait appears to skip generations, although I don't like this terminology.

It skips generations. These are the kinds of properties that you have. So, hemophilia, a good example of this, if I have a child with hemophilia, male with hemophilia, would you be surprised if his uncle had hemophilia? Which uncle would it be, maternal or paternal?

The maternal uncle would have hemophilia most likely.

It's always possible it could be paternal. This is the problem with human genetics is you've got to get enough families so the pattern becomes overwhelmingly clear, OK, because otherwise, as you can see with small numbers, it's tough to be absolutely certain.

So, these are properties of X linked traits.

How about baldness? Is baldness, that's a sex-linked trait? How come? You don't see a lot of bald females.

Does that prove it's sex linked? Sorry? Guys are stressed more.

[LAUGHTER] Is there evidence that it has anything to do with stress?

Actually, it has to do with excess testosterone it turns out, that high levels of testosterone are correlated with male pattern baldness, but does the fact that males become bald indicate that this is a sex linked trait? No. Just because it's predominant in male, we have to check these other properties.

Is it the case that bald fathers tend to have bald sons?

Any evidence on this point? Common-sensical evidence from observation? It's pretty clear. It's very clearly not a sex-linked trait. It's a sex-limited trait, because in order to show this you need to be male because the high levels of testosterone are not found in females even if they have the genotype that might predispose them to become bald if they were male. So, it actually is not a sex-linked trait at all, and it's very clear that male pattern baldness does run in families more vertically. So, you've got to be careful about the difference between sex linked and sex limited, and sex linked you can really pick out from transmission and families.

OK, here's another one. New pedigree.

She married twice here. OK, what do we got?

Yep? She married again. She married twice. She didn't have any offspring the second time. But that happens, and you have to be able to draw it in the pedigree.

She's entitled, all right. OK, so she got married again, no offspring from this marriage. That's her legal symbol. You guys think that's funny. It's real, you know?

OK, that doesn't mean she's married to two people at the same time.

This is not a temporal picture. So, what do we got here? Yep?

Sorry, of this person? Well, I'm drawing them as an empty symbol here, indicating that we do not think they have the trait.

They're not carriers. How do you propose to find that out?

Look at the children. Well, the children are affected. They could be carriers. The data are what they are.

You've got to interpret it. Does this person have to be a carrier? What kind of trait do you think this is?

Dominant? Does this look like autosomal dominant to you?

Yep? Oh, not all the kids have the trait in the first generation, and if this was dominant, they'd all have it? What's a possible genotype for this person?

Mutant over plus. And, these kids could be mutant over plus.

This could be plus over plus, and this could be plus over plus, mutant over plus, plus over plus, mutant over plus, and plus over plus would be one possibility. On average, what fraction of the kids should get the trait? About half the kids, right? So, let's see what characteristics we have here. We see the trait in every generation.

On average, half the kids get the trait.

Half of the offspring of an affected individual are affected.

What else? Males and females? Roughly equal in males and females?

Sorry? One, two, three, four, five to two. So, it's a 5:2 ratio?

Oh, in the offspring it's a 2:1 ratio. So, this is like Mendel.

You see this number and you say, OK, 2:1. Isn't that trying to tell me something? Not with six offspring. That's the problem is with six offspring, 2:1 might be trying to tell you 1:1.

And it is. If I had a dominantly inherited trait where there's a 50/50 chance of each offspring getting the disease and it was autosomal, not sex linked, there would be very good odds of getting two males and one female because it happens: flip coins and it happens. So, you have to take that into account, and here you see what else we have. Roughly equal numbers of males and females, they transmit equally, and unaffecteds never transmit.

This would be the classic autosomal dominant trait.

Right, here this mutant would go mutant over plus, mutant over plus, plus over plus, mutant over plus, plus over plus, plus over plus, and you'd see here that three out of the five here, and one, two, three out of the six there: that's a little more than half but it's small numbers here, right? This is a classic autosomal dominant as in the textbooks. Yes? Turns out not to make too much of a difference. It turns out that there's lots of genome that's on either. And so, it is true that males are more susceptible to certain genetic diseases.

So, it'll be some excess, but it won't matter for this.

Now, in real life it doesn't always work so beautifully.

We'll take an example: colon cancer. There are particular autosomal dominant mutations here that cause a high risk of colon cancer.

People who have mutations in a certain gene, MLH-1, have about a 70% risk of getting colon cancer in their life.

But notice, it's not 100%. You might have incomplete penetrance.

Incompletely penetrance means not everybody who gets the genotype gets the phenotype. Not all people with the M over plus genotype show the phenotype. Once you do that, it messes up our picture colossally, because, tell me, how do we know that this person over here is not actually M over plus.

Maybe they're cryptic. They haven't shown the phenotype.

And maybe, it'll appear in the next generation. That'll screw up everything. It screws up our rule about not transmitting through unaffected, it screws up the rule about not being shown in every generation, and it will even screw up our 50/50 ratio because if half the offspring get M over plus, but only 70% of that half show the phenotype, then only 35% of the offspring will show the phenotype. Unfortunately, this is real life.

When human geneticists really look at traits, many mutations, most except the most severe are incompletely penetrant.

And so you have to really begin to gather a lot of data to demonstrate that you're dealing with an autosomal dominant trait that's incompletely penetrant. And then there are other issues.

There's a gene on chromosome number 17 called BRCA-1, mutations in which predisposed to a very high risk of breast cancer but only in women. Males carry the mutation and do not have breast cancer. There are other mutations that do cause breast cancer in males. Males have breast tissue, and can have breast cancer, but the one on chromosome 17 does not. And so, there you would only see this transmitted through females.

It would skip into males without showing a phenotype, etc. So, in real life, life's a bit more complicated.

All right, so autosomal dominance. Now, let's take one more pedigree.

Sorry? Sex limited, but not sex linked.

So, on chromosome 17, which is a bona fide autosome, but it's sex limited in that phenotype can only show itself in an individual who happens to be female. Yes? Sorry? How come autosomal recessive? So, if that left guy up there is actually a heterozygote, and up there that individual, so if we had a homozygote, homozygote, heterozygote, homozygote, ooh, you can interpret that pedigree if you want to as an autosomal recessive, provided that M is pretty frequent in the population. That's right. Human geneticists, in fact, to really prove that they've got the right model, collect a lot of pedigrees and run a computer model.

The computer model first tries out autosomal recessive, tries out autosomal dominant, tries out dominant with incomplete penetrance, and for every possible model figures out the statistical probability that you would see such data under that model.

And when the data become overwhelming and you say, yeah, with one pedigree, any pedigree I draw on the board, it could actually fit almost any for the models. It doesn't say this in the textbooks, but it's true. I get enough pedigrees, and eventually I say the odds are 105 times more likely that this collection of pedigrees would arise from autosomal dominance, inheritance with incomplete penetrance of about 80%.

Then, from autosomal recessive inheritance, then I get to write a paper about it. That's really what human geneticists do is they have to collect enough, now, any other organism, you'd just set up a cross, but you can't. And, as long as we have nontrivial models, we really have to collect a lot of data. Let's take the next pedigree, great, that you're thinking like a human geneticist.

It's very good. Here's the next pedigree. Actually, I'm going to reverse it. There we go.

What's that? Who knows? You can't tell. Good, I've got you up to training to the point where, but in textbooks, this would be autosomal recessive. Or it could be anything.

You know that, right? But the textbooks would show you this picture as an autosomal recessive. But of course, what else could it be? It could be an autosomal dominant with incomplete penetrance.

It could be sex linked. It could be a lot of things.

It could also be, I haven't told you the phenotype.

What if the phenotype here was getting hit by a truck?

[LAUGHTER] Would you tend to observe this? Yep, so getting hit by a truck, for example, if someone gets hit by a truck, it's unlikely either their parents were hit by a truck, or going back several generations that their grandparents were hit by a truck. So, how do you tell being hit by a truck from, I mean, that is to say, how do you know that something's genetic at all? When it's relatively rare and it pops up in a pedigree, how do you know it's genetic?

Because of the DNA. But, I mean, it takes a lot of work to find the gene and all that as we'll come to the course. You might want a little bit of assurance before you go write the grant to the NIH and say I'm going to find the gene for this because you write it and say I'm going to find the gene for getting hit by a truck, and they're going to write back and say show me that it's worth spending money to find that gene. Show me that it's true. So, what kind of things would we look for? If we wanted to show something was autosomal recessive in a population, what would we do?

More data. So, we collect a lot of families, and what would we see? As we collected more and more families, we begin to see what things? Sometimes we might see families like this, or we might see families like this. [LAUGHTER] If both parents were mutants, all the children would be mutant, right? We'd color them in mutant. Is that true? Well, first off, it depends. Some of the things we want to study are extremely severe medical genetical phenotypes, and they're not going to live to have children. So, that's an issue that you have to deal with. But, it is true that if it was autosomal recessive, a mating between two homozygotes for that gene would transmit. [LAUGHTER] What if they were all in the same car? Which is a very important part, because we joke about the car, but diet, things like that, are familial correlated environmental factors. There are environmental factors that correlate within a family.

And so, it's not trivial to make this point. So, all right, we'll be able to demonstrate what's the real proof of Mendelian inheritance here? Because they could all be in the same car, or they all eat the same kind of food or something like that, which predisposes them a certain way. So, we're going to want some better proofs of these things. How about Mendelian ratios?

Mendelian ratios anyone? No, because it could be incomplete autosomal dominance. I don't want to mess you up.

On the exams, you guys can think cleanly about simple things.

But, this could be dominant with incomplete penetrance, though the TA's are going to hate me because I'm telling you that, anyway, what about Mendelian ratios? How about something that's a pretty good prediction? What fraction of the offspring will be affected? We get a lot of families, line them all up. What fraction of the offspring?

A quarter. Now, that's a hard and fast prediction.

One quarter of the offspring are effective. When I have a mating between two homozygotes, so what am I going to do?

I'm going to go out. I'm going to collect a lot of families.

Maybe I'll collect 100 families because it'll be a particular disease, diastrophic dysplasia or something like that, xeroderma pygmentosa, ataxia teleangiectasia, and I will go to the disease foundation, and I will get all the pedigrees for all the families, and I'll see how many times it was one affected, two affected, three affected, etc. And on average, the proportion affecteds will be a quarter, except it's not true.

If I actually do that, I find that the ratio of affecteds is typically more like a third. It isn't a quarter.

Now, this should disturb you greatly because you know full well that M over plus by M over plus should give you a quarter affecteds.

But when you actually look at human families, it's not.

Why? In other words, when we count up all the matings between heterozygotes, we'll collect all the matings that produce one affected child. We'll collect all the matings that produce two affected children. We'll collect all the matings that produce three affected children. But, we will fail to collect those matings between homozygotes that produce zero affected children.

And so, we will systematically overestimate the proportion.

Of course, what we really have to do is go out and get all of those couples who were both carriers, but because they had a small number of children didn't happen to have an affected child.

That's not very easy to do especially when you don't know the gene in advance. So, when human geneticists try to go out and measure the one-quarter Mendelian ratio, you can't. But what you can do is the following, conditional on the first trial being affected, now what will be the proportion of subsequent children who are affected?

A quarter. If I make it conditional, conditioning on having a first child who's affected, number one child who's affected, then I know I've got a mating between heterozygotes.

Subsequent offspring now do not have that bias.

And so, as a matter of fact, you think this pretty cool thought, right? You've got a condition on one. It turns out there's a very famous paper about cystic fibrosis where somebody forgot this point and made a huge big deal in the literature about the fact that a third of the kids on average had cystic fibrosis in these families, and proposed all sorts of models about how cystic fibrosis might be advantageous and would lead to fertility increases and all that.

In fact, it was just a failure to correct for this little statistical bias. OK, this is what human geneticists do is they've got to deal with the popular, now, there's one other trick that you can use to know that something is autosomal recessive.

That trick is this. To site this trick, I have to go back to a person called Archibald Garrett.

Archibald Garrett was a physician in London around 1900.

Garrett studied children with the trait alkoptonuria.

Alkoptonuria was what alkopton means black. Uria means urine.

They had black urine. This was evident because their urine turned black on treatment with alkaline. How would you treat urine with alkaline. How would people know this? Sorry? Outhouses with lime, yeah, and who's going to look at the children's urine, or something like that? But you're on the right track.

How about diapers? You wash diapers, cloth diapers, in alkaline. They turn black. This was evident from black diapers.

The kids' urine would turn black. So, he observed this, and you know what Garrett noticed is when he studied, children alkoptonuria, he found that a very large fraction of affected offspring were in fact produced from matings of first cousins.

Consanguineous matings: now you laugh, but in fact consanguinity has been something that has been favored in many societies, and in Britain, particularly amongst the upper class in Britain in 1900, marriage or first cousins was quite common, but not as common as he observed. He found that eight out of 17 alkoptonuria patients were the products of first cousin marriages.

That's way off the charts because it's nearly a half, when in fact the typical rate in Britain might have been about 5%.

So, on the basis of that in the early 1900's, Garrett was able to show only a few years after the rediscovery of Mendel's work that this property of recessive traits, enrichment in the offspring of consanguineous marriages, was a clear demonstration of Mendelian inheritance. Not only did he do that, but Garrett knew because of the work of some biochemists, and this is way cool, that the problem with the urine was that these patients put out in their urine a lot of what's called homogentisic acid, HGA, which basically is a phenolic ring. What Garrett did was he, and that stuff turns black on exposure to air. What might produce from the things you've learned already some kind of ring like that?

What building blocks do you know have rings like that of things you've studied already? Phenylalanine, tyrosine both have rings. Suppose somebody had problems breaking down homogentisic acid. Suppose there was some pathway where proteins were broken down into amino acids including phenylalanine and tyrosine. And, they were broken down into homogentisic acid. And they were broken down into I don't know what. And, suppose like we had up there, patients had a mutation in that enzyme. What would happen if I fed patients a lot of protein? In their urine, you would recover lots of homogentisic acid. Suppose I fed them a lot of tyrosine.

I'd get a lot of homogentisic acid because the body couldn't break it down. Suppose I fed them a lot of phenylalanine.

They would excrete a lot of homogentisic acid.

Suppose I fed them homogentisic acid. I would get quantitative amounts of homogentisic acid. Garrett did this. These are the days before institutional review boards, you know, informed consent. It turns out it's harmless feeding them proteins and things like that. But in fact, Garrett, in 1911, worked out that this trait had to be recessive because of its population genetics, and inferred a biochemical pathway by feeding different things along the way and was able to connect a mutation in a gene to a problem with a specific biochemical pathway.

Sorry, 1908: this was his Croonian Lecture in 1908.

Eight years after the rediscovery of Mendel, he's able to connect genetic defect, showing it's genetic by transmission, to biochemical defect showing that he has a pathway that he can feed things into. And, it all blocks up at the inability to metabolize homogentisic acid. He has connected gene to enzyme by 1908. What do you think the reaction to this was? Polite bewilderment, and it sunk like a stone. Nobody was prepared to hear this. This is very much like Mendel in my opinion. Now, he was a distinguished professor.

It was the Croonian Lecture. He got lots of accolades and all that, and people said, what a lovely lecture that was, and proceeded to completely forget this connection between genes and enzymes, genes and proteins. It was not until 40 years later or so that Beadle and Tatum, working with a fungus, actually rosper not yeast, demonstrated that all these mutants interfered with the ability to digest or to make particular amino acids, and wrote this up as the one gene, one enzyme hypothesis of how genes encode enzymes, and won the Nobel Prize for this work, but in fact in their Nobel address, Beetle and Tatum noted, actually, you know, Garrett kind of knew all this. But, people weren't ready, yet, to digest it. Genetics had just come along, Biochemistry had just really been invented in the last ten years, and the idea of uniting genetics and biochemistry was just something people weren't prepared for yet. More next time.


Genetics

Genetics is a science, which deals with the study and understanding of heredity, evolution, development, ecology, molecular biology and forensic science. A German scientist by name Gregor Johann Mendel was a first founder of Genetics, hence he is also known as the father of genetics. He first demonstrated the inheritance of traits in pea plants and later it was referred to as a Mendelian inheritance.
The main concept behind studying genetics are:

  • It explains how the traits are passed from parents to their
  • It explains how the traits are passed from parents to their offsprings.
    It also explains about the gene and the number of chromosomes present in an individual with their importance.

Variation of Species

The term variation can be defined as the range of differences between individual organisms. This variation occurs within the species. For example differences between individual human beings. This variation is of two types:
Continuous Variation: In this variation, all types of features have a normal distribution.
For example Height of individual human beings, the width of a leaf, weight of a cat, etc.
Discontinuous Variation: In this case, there are only a few features, which fall under the category of discontinuous variation.
For example Different color of the flower in a single plant species, different types of blood groups in individual human beings, different types of the ear lobe, etc.

Heredity

The term heredity can be defined as the process of transferring the characters or the traits from the parents to their offspring. In this process of transformation, the characters get transferred through the genes present in the DNA. These DNA’s are present in the chromosomes, which are present in the nucleus of the cell. The transferring of the characters includes the contribution of the equal amount of genetic material from both the parents (mother and father) to their offspring.

Gene Expression

Before moving into gene expression, let us know something about a gene.
What is a Gene?
A gene is a segment of the DNA, which contains all sorts of information that are required for coding some important functions. Hence a gene is called as a unit of information. Every cell in an organism has a similar set of genes. A gene is transferred from the parents to their offsprings. The term gene was coined by the scientist named Johann sen in the year 1909.
Gene expression is the activation of a gene, which results in a protein. Gene expression is the process by which the genetic information is used in the synthesis of a gene. This process is generally used in all eukaryote, prokaryotes and in viruses for generating the macromolecular machinery of life.

Genetic Code

A code that contains all types of genetic information, which are present in the nucleotide sequences of DNA or RNA and are later translated into proteins by the living cells. These codes can be expressed either as RNA codons or DNA codons. This code instructs a gene to guide the cell to make a specific protein. A, T, G, C is the alphabetical letter of the DNA code. These letters stand for adenine, thymine, guanine and cytosine, which completely make up the nucleotide bases of DNA. Each and every code combines with these four chemicals for the synthesis of proteins.

DNA Structure

DNA is called the blueprint of life as it contains all types of instructions for making proteins within the cell. It is a very long polymer and the basic shape of the DNA is just like a twisted ladder. Hence it is also called as a double helices DNA. The backbone of the DNA molecule is alternating phosphates, deoxy Ribose sugar, and nitrogenous bases. The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T).

DNA molecule

Genetic Inheritance

It can be defined as the transformation of traits from the parents to their offspring. Everyone has two copies of every gene among which one inherited from the mother cell and another one is inherited from the father cell.
Causes of Variation and Evolution.
There are mainly two factors, which are responsible for variation and evolution of species. They are:
Genetic variation: In this case, the difference in variation is mainly due to the presence of DNA inside the cells of an organism. The features of each species will be different in their color, size, and shape because they come from the parent with different alleles, which codes for different characteristics.
It can be defined as the transformation of traits from the parents to their offspring. Everyone has two copies of every gene among which one inherited from the mother cell and another one is inherited from the father cell.
Causes of Variation and Evolution.
There are mainly two factors, which are responsible for variation and evolution of species. They are:
Genetic variation: In this case, the difference in variation is mainly due to the presence of DNA inside the cells of an organism. The features of each species will be different in their color, size, and shape because they come from the parent with different alleles, which codes for different characteristics.


Genetics (Science 1.9)

Welcome to the topic of genetics! Genetics is the study of heredity. Heredity is a biological process where a parent passes certain genes onto their children or offspring. Every child inherits genes from both of their biological parents and these genes in turn express specific traits. Some of these traits may be physical for example hair and eye color and skin color etc. On the other hand some genes may also carry the risk of certain diseases and disorders that may pass on from parents to their offspring. This topic is like a new language for most so stick at it. Learn the key words, watch the videos and listen to your teachers!

Fantastic PowerPoint and resources from GZScience online here

Before you start work through the following activities from DNA from the beginning.

No brain to small - This website has some amazing!! resources - go have a look here

Below are the key parts to know:

Nucleus is the Organelle in a cell that contains DNA.

Deoxyribonucleic Acid is a self-replicating molecule present in nearly all living organisms. It’s what the chromosomes are made up of.

The Chromosome is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells. They carrying genetic information in the form of genes. Chromosomes are made up of long lengths of DNA.

Chromosomes Location: Contained within the nucleus

Made up of: DNA (nucleic acids – a phosphate, sugar and base) with various binding proteins holding it together

Function (what it does): Containing genetic information to enable an organism to manufacture all the proteins required to develop and maintain an organism when necessary.

A Gene is a short length of DNA that carries the genetic code for a particular characteristic or cell activity. Different forms of the same gene are called Alleles. They can be dominant or recessive.

A trait is a genetically determined characteristic such as eye colour or hair colour.

Benjamin Himme's epic video on DNA, she is a long one!

DNA is the heredity material of the cell which is found in the chromosomes in the nucleus. These are found as strands each one of these strands of DNA is called a chromosome. A gene is a segment of DNA, found in a small section of the chromosome. Along the DNA, base sequences provide the code for building different proteins, which then determine particular features. Slight differences in the sequence of the bases making up a gene are called alleles and they cause the variations in the phenotypes. These differences lead to genetic variation between individuals.

From 2013's exam.. Chromosomes are made up of DNA. DNA is a large molecule that is coiled into a double helix (twisted ladder structure). It is responsible for determining the phenotype of an organism. Along this molecule are bases. These bases pair up A always pairs with T, and G with C.

A sequence of bases which codes for a particular trait (eg, eye colour) is called a gene.

The different versions of each gene are called alleles, and these show the different variations of each characteristic, eg brown / blue eyes. Because chromosomes come in pairs for each trait, there will be two possible alleles. These different versions of genes (alleles) occur as the DNA base sequence is different.

This combination of alleles for each trait is called the genotype this can be any combination of two of the available alleles. The genotype determines the phenotype (the physical appearance) of the organism. Whichever alleles are present may be expressed. Dominant alleles (B) will be expressed over recessive alleles (b).

DNA from the Beginning Good introduction to DNA and genetics

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).

The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.

DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.

The shape of DNA at the molecular level is thought to look like a gently twisting ladder. Each of the rungs on the ladder represents a chemical bond between the chemicals that make up the DNA molecule. These chemicals are called nucleotides and include: (click on image to make bigger)

DNA is made from Deoxyribonucleic Acid. DNA is called a polymer because it is made up of many repeating units called nucleotides.

DNA strands are loose within the nucleus of a cell. Just prior to cell division the DNA folds up around proteins called histones into tight coils, then into structured chromosomes. The human cell has 46 chromosomes arranged into 23 pairs of chromosomes. Each chromosome in a pair has the same genes, called homologous pairs – except the sex chromosome pair – although there may be variation between the genes of each pair, as one comes from the father and one comes from the mother.

Chromosomes are made up of long lengths of DNA arranged in a twisted ladder.

The base pairing rules mean that guanine (G) always bonds to cytosine (C), and thymine (T) always bonds to adenine (A).

A gene is a section of DNA that carries the genetic code for a particular characteristic. An allele is an alternative form of a gene. They can be dominant or recessive.

During fertilisation a person gets two different alleles for the same gene because one allele is inherited from your mother, the other comes from your father.

A great video that covers this in more detail than is required a L1. You need to know where it happens, why it happens and the basics steps that are below.

The original DNA strand unwinds as the bonds between the bases break.

New nucleotides are brought in. They bond with the bases on the original DNA strand according to the base-pairing rules.

Once the new nucleotides have bonded, the DNA molecule begins to coil back up into a double helix. At the end of the process, two new strands of DNA are produced. Both are exact copies of the original strand.

Chromosomes come in pairs. One pair is the sex chromosomes – XX in females and XY in males. A complete set of chromosomes of an organism placed into pairs of matching chromosomes is called a karyotype. The human karyotype consists of 23 pairs of chromosomes

Genotype is the genetic make-up of an individual organism. Your genotype functions as a set of instructions for the growth and development of your body. The word ‘genotype’ is usually used when talking about the genetics of a particular trait (like eye colour).

Phenotype is the observable physical or biochemical characteristics of an individual organism, determined by both genetic make-up and environmental influences, for example, height, weight and skin colour.

A codon is a group of three bases that code for an specific amino acid.DNA contains the instructions for linking amino acids. These amino acids join together to make proteins. Proteins are important because they are the building blocks of our body and carry out many important functions within the body. The base sequence of DNA can be broken down into codons (three-letter sequences). One codon codes for one amino acid.

Homologous pairs are chromosomes that have the same genes.

Biological concepts and processes relating to variation in phenotype will be selected from:

An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome.

Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked.

The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).

Organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. The dominant allele is expressed and the recessive allele is masked. Using the previous example, round seed shape (R) is dominant and wrinkled seed shape (r) is recessive. Round: (RR) or (Rr), Wrinkled: (rr).

A mutation is a change in the base sequence of DNA caused by a mutagen. A mutagen is an agent, such as a chemical substance, UV light or radiation, that causes genetic mutation.

Mutation is a permanent / random changes in the DNA/ genetic material. Mutation must occur in gamete-producing cells to enter the gene pool of the population

A mutation is a permanent (unrepaired) change in an organisms DNA.

They introduce new alleles into a population. Most mutations are harmful.

Mutations are caused by mutagens.

Beneficial ones tend to occur more often in organisms with short generation times.

Many may be silent – not observed – and may only be selected for or against at a later date.

Neutral mutations make no change at all.

Beneficial mutation = A mutation that gives an organism a survival advantage.

Harmful mutation = A mutation that effects the survival of the organism.

Silent mutation = a mutation which has no observable effect on the organism.

(you are not required to provide the names of the stages of meiosis)

Sex cells have one set of chromosomes body cells have two. Click here to work through an animation

Online simulator for Mitosis and Meiosis Click here

Meiosis is a type of cell division that occurs in the testes (males) and ovaries (females). It produces four new cells (gametes) that are genetically different to each other, and to the parent cell. They contain half the number of chromosomes that are in the parent cell.Meiosis leads to genetic variation via two processes. When homologous pairs of chromosomes line up during meiosis, they do so randomly. This means it is completely random which combination of alleles end up in a particular gamete. This process is called independent assortment.The second way meiosis leads to genetic variation is via a process called crossing over. This occurs when homologous pairs of chromosomes line up at the cell equator and swap sections of genetic material, and therefore alleles. Because of crossing over, each gamete will contain different combinations of alleles.

Meiosis Explained

Mitosis explained (in more detail than you need at L1)

(in producing a new mix of alleles)

With sexual reproduction two individuals contribute genetic material with traits generally being determined by the two alleles for each gene. The process of meiosis which creates the gametes and recombination leads to an individual with a genetic make-up that differs from both parents. Over time that process allows the movements of alleles from one population to the next.

• Gametes are sex cells (sperm and egg) which are formed in the testes and ovaries. During gamete formation (meiosis), the homologous chromosomes are halved and the gamete will inherit one of each pair of chromosomes. Which chromosome is passed on is random due to the process of independent assortment.

• During fertilisation, the gametes combine and the resulting offspring will have two alleles – they may inherit two alleles the same, homozygous, and show that characteristic or they may inherit one of each allele, heterozygous in which case they will show the dominant allele in their phenotype. Genetic variation: variety within a population, eg different alleles possible for each gene. The advantage of variation to a population is that it may see some individuals survive if environment changes, in this case if drought occurs. Because of variation, not all individuals will be wiped out. Those with favourable alleles / traits / phenotypes will survive and be able to pass on genetic material to offspring and therefore survival of the species occurs.

• Possible disadvantages: need two parents that are able to reproduce, if conditions are stable could introduce variation, which may be counterproductive.

The patterns of inheritance involving simple monohybrid inheritance showing complete dominance

If you work through the student part of this animation series you will know everything you need to on monohybrid crosses.


Introduction

The three letters “DNA” have now become associated with crime solving, paternity testing, human identification, and genetic testing. DNA can be retrieved from hair, blood, or saliva. With the exception of identical twins, each person’s DNA is unique and it is possible to detect differences between human beings on the basis of their unique DNA sequence.

DNA analysis has many practical applications beyond forensics and paternity testing. DNA testing is used for tracing genealogy and identifying pathogens. In the medical field, DNA is used in diagnostics, new vaccine development, and cancer therapy. It is now possible to determine predisposition to many diseases by analyzing genes.

DNA is the genetic material passed from parent to offspring for all life on Earth. The technology of molecular genetics developed in the last half century has enabled us to see deep into the history of life to deduce the relationships between living things in ways never thought possible. It also allows us to understand the workings of evolution in populations of organisms. Over a thousand species have had their entire genome sequenced, and there have been thousands of individual human genome sequences completed. These sequences will allow us to understand human disease and the relationship of humans to the rest of the tree of life. Finally, molecular genetics techniques have revolutionized plant and animal breeding for human agricultural needs. All of these advances in biotechnology depended on basic research leading to the discovery of the structure of DNA in 1953, and the research since then that has uncovered the details of DNA replication and the complex process leading to the expression of DNA in the form of proteins in the cell.

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    Developmental Genetics

    Research opportunities in developmental genetics include the examination of mechanisms that control embryonic and post-embryonic development, cell-cell signaling, transcriptional patterning, stem cells, cell differentiation, and morphogenesis. These processes are studied in model systems for animal and plant development, including Drosophila, C. elegans, ascidians and Arabidopsis. Researchers in this area are part of a joint Developmental Systems Graduate program with members of the Skirball Institute at NYU Medical School. This joint graduate program is supported by a training grant from the NIH, and provides a formal vehicle for interactions with faculty and students studying other developmental systems including vertebrates. Regional meetings of Drosophila, C. elegans and other model system researchers are hosted by NYU Biology, and attract researchers from the broad New York area.

    Research in Develpmental Genetics is conducted by faculty in the Center for Developmental Genetics, located in the Brown Building on floors 9 & 10 and headed by Dr. Claude Desplan.


    Acknowledgments

    The authors thank all researchers and clinicians for their contributions to the field and apologize to those whose work they could not describe or cite.

    The laboratory of K.D.K. was supported by a European Research Council (ERC) starting grant (334946), Fonds Wetenschappelijk Onderzoek (FWO)-Vlaanderen funding (G067015N and G084013N), and a Stichting Tegen Kanker grant (2012-176). The laboratory of J.C. was supported by an ERC consolidator grant (617340), and funding from FWO-Vlaanderen (G.0683.12), Stichting Tegen Kanker (2014-120), and Kom Op Tegen Kanker. T.G. was supported by the Emmanuel van der Schueren Kom Op Tegen Kanker fellowship.


    Watch the video: mitosis 3d animation Phases of mitosiscell division (September 2022).


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