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7.6: Sex Chromosomes - Biology

7.6: Sex Chromosomes - Biology


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The nuclei of human cells contain 22 autosomes and 2 sex chromosomes. In females, the sex chromosomes are the 2 X chromosomes. Males have one X chromosome and one Y chromosome. The presence of the Y chromosome is decisive for unleashing the developmental program that leads to a baby boy.

The Y Chromosome

In making sperm by meiosis, the X and Y chromosomes must separate in anaphase just as homologous autosomes do. This occurs without a problem because, like homologous autosomes, the X and Y chromosome synapse during prophase of meiosis I. There is a small region of homology shared by the X and Y chromosome and synapsis occurs at that region.

This image, shows synapsis of the X and Y chromosomes of a mouse during prophase of meiosis I. Crossing over occurs in two regions of pairing, called the pseudoautosomal regions. These are located at opposite ends of the chromosome.

The Pseudoautosomal Regions

The pseudoautosomal regions get their name because any genes located within them (so far only 9 have been found) are inherited just like any autosomal genes. Males have two copies of these genes: one in the pseudoautosomal region of their Y, the other in the corresponding portion of their X chromosome. So males can inherit an allele originally present on the X chromosome of their father and females can inherit an allele originally present on the Y chromosome of their father.

Genes outside the pseudoautosomal regions

Although 95% of the Y chromosome lies between the pseudoautosomal regions, only 27 different functional genes have been found here. Over half of this region is genetically-barren heterochromatin. Of the 27 genes found in the euchromatin, some encode proteins used by all cells. The others encode proteins that appear to function only in the testes. A key player in this latter group is SRY.

SRY

SRY (for sex-determining region Y) is a gene located on the short (p) arm just outside the pseudoautosomal region. It is the master switch that triggers the events that converts the embryo into a male. Without this gene, you get a female instead.

What is the evidence?

  1. On very rare occasions aneuploid humans are born with such karyotypes as XXY, XXXY, and even XXXXY. Despite their extra X chromosomes, all these cases are male.
  2. This image shows two mice with an XX karyotype (and thus they should be female). However, as you may be able to see, they have a male phenotype. This is because they are transgenic for SRY. Fertilized XX eggs were injected with DNA carrying the SRY gene.

Although these mice have testes, male sex hormones, and normal mating behavior, they are sterile.

  1. Another rarity: XX humans with testicular tissue because a translocation has placed the SRY gene on one of the X chromosomes
  2. Still another rarity that demonstrates the case: women with an XY karyotype who, despite their Y chromosome, are female because of a destructive mutation in SRY.

In 1996, a test based on a molecular probe for SRY was used to ensure that potential competitors for the women's Olympic events in Atlanta had no SRY gene. But because of possibilities like that in case 4, this testing is no longer used to screen female Olympic athletes.

The X Chromosome

The X chromosome carries nearly 1,000 genes but few, if any, of these have anything to do directly with sex. However, the inheritance of these genes follows special rules. These arise because:

  • males have only a single X chromosome
  • almost all the genes on the X have no counterpart on the Y; thus
  • any gene on the X, even if recessive in females, will be expressed in males.

Genes inherited in this fashion are described as sex-linked or, more precisely, X-linked.

X-Linkage example

Hemophilia is a blood clotting disorder caused by a mutant gene encoding either

  • clotting factor VIII, causing hemophilia A or
  • clotting factor IX, causing hemophilia B.

Both genes are located on the X chromosome (shown here in red). With only a single X chromosome, males who inherit the defective gene (always from their mother) will be unable to produce the clotting factor and suffer from difficult-to-control episodes of bleeding. In heterozygous females, the unmutated copy of the gene will provide all the clotting factor they need. Heterozygous females are called "carriers" because although they show no symptoms, they pass the gene on to approximately half their sons, who develop the disease, and half their daughters, who also become carriers.

XY
XXXXY
XhXhXXhY

Women rarely suffer from hemophilia because to do so they would have to inherit a defective gene from their father as well as their mother. Until recently, few hemophiliacs ever became fathers.

X-chromosome Inactivation (XCI)

Human females inherit two copies of every gene on the X chromosome, whereas males inherit only one (with some exceptions: the 9 pseudoautosomal genes and the small number of "housekeeping" genes found on the Y). But for the hundreds of other genes on the X, are males at a disadvantage in the amount of gene product their cells produce? The answer is no, because females have only a single active X chromosome in each cell.

During interphase, chromosomes are too tenuous to be stained and seen by light microscopy. However, a dense, stainable structure, called a Barr body (after its discoverer) is seen in the interphase nuclei of female mammals. The Barr body is one of the X chromosomes. Its compact appearance reflects its inactivity. So, the cells of females have only one functioning copy of each X-linked gene — the same as males.

X-chromosome inactivation occurs early in embryonic development. In a given cell, which of a female's X chromosomes becomes inactivated and converted into a Barr body is a matter of chance (except in marsupials like the kangaroo, where it is always the father's X chromosome that is inactivated). After inactivation has occurred, all the descendants of that cell will have the same chromosome inactivated. Thus X-chromosome inactivation creates clones with differing effective gene content. An organism whose cells vary in effective gene content and hence in the expression of a trait, is called a genetic mosaic.

Mechanism of X-chromosome inactivation

Inactivation of an X chromosome requires a gene on that chromosome called XIST.

  • XIST is transcribed into a long noncoding RNA.
  • XIST RNA accumulates along the X chromosome containing the active XIST gene and proceeds to inactivate all (or almost all) of the hundreds of other genes on that chromosome.
  • Barr bodies are inactive X chromosomes "painted" with XIST RNA.

The Sequence of Events in Mice

  • During the first cell divisions of the female mouse zygote, the XIST locus on the father's X chromosome is expressed so most of his X-linked genes are silent.
  • By the time the blastocyst has formed, the silencing of the paternal X chromosome still continues in the trophoblast (which will go on to form the placenta) but
  • in the inner cell mass (the ICM, which will go on to form the embryo) transcription of XIST ceases on the paternal X chromosome allowing its hundreds of other genes to be expressed. The shut-down of the XIST locus is done by methylating XIST regulatory sequences. So the pluripotent stem cells of the ICM express both X chromosomes.
  • However, as embryonic development proceeds, X-chromosome inactivation begins again. But this time it is entirely random. There is no predicting whether it will be the maternal X or the paternal X that is inactivated in a given cell.

Some genes on the X chromosome escape inactivation

What about those 18 genes that are found on the Y as well as the X? There should be no need for females to inactivate one copy of these to keep in balance with the situation in males. And, as it turns out, these genes escape inactivation in females. Just how they manage this is still under investigation.

X-chromosome Abnormalities

As we saw above, people are sometimes found with abnormal numbers of X chromosomes. Unlike most cases of aneuploidy, which are lethal, the phenotypic effects of aneuploidy of the X chromosome are usually not severe.

Examples:

  • Females with but a single intact X chromosome (usually the one she got from her mother) in some (thus a genetic mosaic) or all of her cells show a variable constellation of phenotypic traits called Turner syndrome. For those girls that survive to birth, the phenotypic effects are generally mild because each cell has a single functioning X chromosome like those of XX females. Number of Barr bodies = zero.
  • XXX, XXXX, XXXXX karyotypes: all females with mild phenotypic effects because in each cell all the extra X chromosomes are inactivated. Number of Barr bodies = number of X chromosomes minus one.
  • Klinefelter's syndrome: people with XXY or XXXY karyotypes are males (because of their Y chromosome). But again, the phenotypic effects of the extra X chromosomes are mild because, just as in females, the extra Xs are inactivated and converted into Barr bodies.

Sex Determination in Other Animals

Although the male fruit fly, Drosophila melanogaster, is X-Y, the Y chromosome does not dictate its maleness but rather the absence of a second X. Furthermore, instead of females shutting down one X to balance the single X of the males — as we do — male flies double the output of their single X relative to that of females.

In birds, moths, schistosomes, and some lizards, the male has two of the same chromosome (designated ZZ), whereas the female has "heterogametic" chromosomes (designated Z and W). In chickens, a single gene on the Z chromosome (designated DMRT1), when present in a double dose (ZZ), produces males while the presence of only one copy of the gene produces females (ZW).

Environmental Sex Determination

In some cold-blooded vertebrates such as

  • fishes
  • reptiles (e.g. certain snakes, lizards, turtles, and all crocodiles and alligators)
  • invertebrates (e.g. certain crustaceans),

sex is determined after fertilization — not by sex chromosomes deposited in the egg.

The choice is usually determined by the temperature at which early embryonic development takes place.

  • In some cases (e.g. many turtles and lizards), a higher temperature during incubation favors the production of females.
  • In other cases (e.g., alligators), a higher temperature favors the production of males.

Even in cases (e.g. some lizards) where there are sex chromosomes, a high temperature can convert a genotypic male (ZZ) into a female.

Hermaphrodites

Hermaphrodites have both male and female sex organs. Many species of fish are hermaphroditic.

Some start out as one sex and then, in response to stimuli in their environment, switch to the other.

Other species have both testes and ovaries at the same time (but seldom fertilize themselves). However, populations of C. elegans consist mostly of hermaphrodites and these only fertilize themselves.

Hermaphroditic fishes have no sex chromosomes.


Sex chromosome

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Sex chromosome, either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of human beings and other mammals are designated by scientists as X and Y. In humans the sex chromosomes consist of one pair of the total of 23 pairs of chromosomes. The other 22 pairs of chromosomes are called autosomes.

Individuals having two X chromosomes (XX) are female individuals having one X chromosome and one Y chromosome (XY) are male. The X chromosome resembles a large autosomal chromosome with a long and a short arm. The Y chromosome has one long arm and a very short second arm. This path to maleness or femaleness originates at the moment of meiosis, when a cell divides to produce gametes, or sex cells having half the normal number of chromosomes. During meiosis the male XY sex-chromosome pair separates and passes on an X or a Y to separate gametes the result is that one-half of the gametes (sperm) that are formed contains the X chromosome and the other half contains the Y chromosome. The female has two X chromosomes, and all female egg cells normally carry a single X. The eggs fertilized by X-bearing sperm become females (XX), whereas those fertilized by Y-bearing sperm become males (XY).

Unlike the paired autosomes, in which each member normally carries alleles (forms) of the same genes, the paired sex chromosomes do not carry an identical complement of genetic information. The X chromosome, being larger, carries many more genes than does the Y. Traits controlled by genes found only on the X chromosome are said to be sex-linked (see linkage group). Recessive sex-linked traits, such as hemophilia and red–green colour blindness, occur far more frequently in men than in women. This is because the male who inherits the recessive allele on his X chromosome has no allele on his Y chromosome to counteract its effects. The female, on the other hand, must inherit the recessive allele on both of her X chromosomes in order to fully display the trait. A woman who inherits the recessive allele for a sex-linked disorder on one of her X chromosomes may, however, show a limited expression of the trait. The reason for this is that, in each somatic cell of a normal female, one of the X chromosomes is randomly deactivated. This deactivated X chromosome can be seen as a small, dark-staining structure—the Barr body—in the cell nucleus.

The effects of genes carried only on the Y chromosome are, of course, expressed only in males. Most of these genes are the so-called maleness determiners, which are necessary for development of the testes in the fetus.

Several disorders are known to be associated with abnormal numbers of sex chromosomes. Turner’s syndrome and Klinefelter’s syndrome are among the most common of these. See also X trisomy XYY-trisomy.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers, Senior Editor.


Sex equality

Different species have developed strikingly different strategies to deal with disparities in the dose of X chromosome between males and females: in XX female mammals, one of the two X chromosomes is randomly inactivated XX hermaphrodite nematodes halve the expression from each X chromosome and male Drosophila double expression from their single X chromosome in somatic cells [2, 3]. These dosage compensation mechanisms serve to balance the differences between the number of copies of X-linked genes in somatic tissues of the two sexes.

"Although we now know that these species use different approaches to achieve dosage compensation, this amounts mainly to playing differently with a limited panoply of chromatin-based modifications," says Philip Avner from the Pasteur Institute in Paris, France. X inactivation in mammals requires expression of the Xist gene, which produces a large, non-coding RNA that coats the inactive X chromosome [3]. The inactive X is characterized by DNA methylation, histone hypoacetylation, late replication and enrichment in the variant histone macroH2A. The hypertranscription of the Drosophila X chromosome in somatic cells is dependent on the 'male specific lethal' (msl) loci, which encode a histone-modifying MSL complex that acetylates histone H4 on lysine 16 (H4 K16) [4].

"Much less attention has been paid to the question of X/autosome dosage," says Avner. "X inactivation would be expected to lead to halved quantities of X-linked gene products compared to autosomal gene products. Haploinsufficiency for the entire X would a priori be expected to be catastrophic to the organism and lead to lethality during early embryonic development." Haploinsufficiency was the issue that Gupta and colleagues set out to address. "There was a lack of evidence for a germline dosage compensation machinery," notes Gupta, citing studies showing that in Drosophila the X chromosomes are not coated with MSL complexes or hyperacetylated on H4 K16 in male germ cells [5] (see the 'Behind the scenes' box for more on the rationale for the work). "Also, Parisi et al. [6] showed that a subset of ribosomal protein encoding genes are equally expressed in both testis and ovaries and we had seen that XAA and XXAA tumors showed very similar gene expression profiles," adds Gupta. "I've been thinking about this problem since I was a graduate student in the late 80s," recalls Brian Oliver who heads the research group at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, USA. "Until microarrays appeared, we didn't really see a good way to do a convincing experiment."


Taiwanese Frog Has Six Sex Chromosomes, Study Shows

Biologists have found a multiple sex chromosome system comprising three different chromosome pairs in Odorrana swinhoana, a species of medium to large-sized frog in the family Ranidae commonly known as the Swinhoe’s brown frog and the Bangkimtsing frog, this species is endemic to Taiwan and widely distributed in elevated areas below 2,000 m.

A graphical abstract showing the six sex chromosomes found in Odorrana swinhoana. Image credit: Ikuo Miura.

Sex chromosomes generally evolve from an ordinary autosomal pair after acquiring a sex-determining gene and thus are composed of a pair of X and Y chromosomes in an XX-XY system or of Z and W chromosomes in a ZZ-ZW system.

Rarely, the sex-chromosome is fused with an autosome and generates multiple sex-chromosome systems.

If either a homolog of a sex-chromosome pair is fused with an autosome, the number of sex-chromosomes increases, while if they are both homologs, the pair of sex-chromosomes remains the same but gets larger in size.

The latter is the case in placental mammals, including humans, in which an autosome corresponding to kangaroo chromosome 5 was fused with both the X and Y chromosomes.

In amphibians, a case of multiple sex-chromosomes is very rare. Generally, the karyotypes (collection of chromosomes) are highly conserved, with little rearrangement among species.

In 1980, a Japanese biologist discovered a male-specific translocation — a chromosomal abnormality that happens when a chromosome breaks and its fragment fuses to another — between two chromosomes in Rana narina (a synonym for Odorrana swinhoana).

That was the first report of multiple sex chromosomes in amphibians, and the sex chromosomes were described as ♂X1Y1X2Y2-♀X1X1X2X2.

The finding suggested that the translocation occurred between the two members of the potential sex-determining chromosomes.

At that time, however, the identification of the chromosomes involved in the translocation was uncertain.

In a new study, Dr. Ikuo Miura of the Hiroshima University’s Amphibian Research Center and colleagues set out to confirm the male-specific translocation and precisely identify the chromosomes involved in the translocation.

They re-investigated the somatic chromosomes as well as meiotic chromosomes of Odorrana swinhoana using chromosome banding and molecular mapping techniques.

Unexpectedly, the translocation was found not to be a single but a triple one, comprising potential sex-chromosomes that include orthologs of the sex-determining genes in mammals, birds and fishes.

The male-specific three translocations created a system of six sex-chromosomes, ♂X1Y1X2Y2X3Y3-♀X1X1X2X2X3X3.

The researchers found the Dmrt1, the male determining gene in birds, and Amh, the male determining gene in fish and platypus, on the Y1 chromosome the Sox3, the ancestral gene of SRY in therian mammals and the male determining gene in medaka fish, on the Y3 chromosome and an unidentified sex-determining gene on the Y2 chromosome.

“Up to now sex chromosome-autosome fusion has been documented as a chance occurrence,” Dr. Miura said.

“In fact, it was like that in this frog, too. The break and fusion of the chromosomes may have occurred by chance.”

But the scientists believe that the chromosome members involved in the fusions were selected non-randomly or inevitably chosen as they probably share a common genomic region.

“To be so, the three may share a common DNA sequence on each of them, which makes them closely localized to each other, and this makes it possible to join the simultaneously occurring breakages and translocations,” Dr. Miura said.

“This rare case suggests sex-specific, nonrandom translocations and thus provides a new viewpoint for the evolutionary meaning of the multiple sex chromosome system.”

“Identifying the genomic sequence common to the potential sex chromosomes would improve understanding of the mechanisms of its evolution and turnover.”

The team’s findings were published in the journal Cells.

Ikuo Miura et al. 2021. Evolution of a Multiple Sex-Chromosome System by Three-Sequential Translocations among Potential Sex-Chromosomes in the Taiwanese Frog Odorrana swinhoana. Cells 10 (3): 661 doi: 10.3390/cells10030661


The 6 Most Common Biological Sexes in Humans

Many of you have expressed an interest in more of my personal essays the documents I use myself to study various topics and take advantage of the so-called “orangutan theory”, which states that forcing yourself to write down your ideas, or speaking them out loud, even if your only audience is a large primate in a circus tent, shifts your brain into a logical mode that gives you a better understanding of what you believe, both inherently and explicitly. I decided to share another one of the in-progress essays, though I modified it to read better online as if it were addressed to the blog audience by changing a small amount of the verbiage. Again, just like my previous essay on religious beliefs manifesting through time, culture, and geographic distance, this is a work in progress that will change substantially by the time I stamp “concluded’ on it and feel as if I really have a handle on the subject matter. It was not originally intended for public consumption as its sole purpose is for me to understand how the various components are connected.

There was a news story about a 66 year old man who discovered, during a trip to the doctor, that he was really a woman. If you don’t have a biology or genetics education background, or never really took an interest in reproductive strategies of various animals and plants in nature, that might seem absurd, or even impossible. Of course, it’s not. It’s far more common than the general population realizes.

The Journal of the Royal Society of Medicine points out that one of the first modern cases came from the 1936 Olympics, hosted by Adolf Hitler. An American named Stella Walsh, commonly called “Stella the Fella”, crushed the competition. She always changed by herself and had muscle tissue and facial features that resembled a man. The Olympic committee did an examination during which the members found that Stella was, in fact, both male and female. Sort of. She had ambiguous genitalia and it was impossible to determine her biological sex. This remained a secret until Stella’s death in 1980 when “she was shot and killed in the cross-fire of an armed bank robbery in Los Angeles”.

Today, we have genetics and DNA that allows us to examine karyotype. We know, without question, that humans are not just born male and female. There are at least six biological sexes that can result in fairly normal lifespans. (There are actually many more than six but they result in spontaneous abortion as the body knows the fetus won’t be viable so it is flushed out of the system in a natural process meant to minimize the amount of nutrients and metabolism devoted to growing non-viable offspring.)

The Six Most Common Karyotypes

The six biological karyotype sexes that do not result in death to the fetus are:

  • X – Roughly 1 in 2,000 to 1 in 5,000 people (Turner’s )
  • XX – Most common form of female
  • XXY – Roughly 1 in 500 to 1 in 1,000 people (Klinefelter)
  • XY – Most common form of male
  • XYY – Roughly 1 out of 1,000 people
  • XXXY – Roughly 1 in 18,000 to 1 in 50,000 births

When you consider that there are 7,000,000,000 alive on the planet, there are almost assuredly tens of millions of people who are not male or female. Many times, these people are unaware of their true sex. It’s interesting to note that everyone assumes that they, personally, are XY or XX. One study in Great Britain showed that 97 out of 100 people who were XYY had no idea. They thought they were a traditional male and had few signs otherwise.

Even today, we irrationally, and rather stupidly, think of someone as a “man” if they look masculine and as a “woman” if the look feminine. It’s entirely arbitrary and can lead to some significant misunderstandings of how the world actually works.

It Is Possible for Your Brain, Your Body, and Your Reproductive Systems to All Have Different Biological Sexes

What makes it even more complicated is that you cannot rely on karyotype alone to determine biological sex. A few years ago, there was a story about a teenage boy who was, in all regards, perfectly normal. He looked male, he acted male, he had a fully functional male reproductive system. He suddenly became extremely sick. He was growing sicker and could have died when it was discovered that he also had a female reproductive system internally. When he menstruated once a month, the excess blood had nowhere to go since there was no available external exit, causing it to be reabsorbed into his body. This boy was male. However, he was also female. It is a gross simplification to act as if he were just a boy. He was more.

Even rarer are the cases of chimeras such as Lydia Fairchild, who have multiple sets of DNA in their body so that they are not the biological parents of their own children, even when conceived through regular reproduction and birthed entirely naturally.

The Case of Riley Grant

And then we get into the really interesting territory. It is possible that your body, your brain, and your reproductive system could all be different biological sexes, or in some cases, biologically one sex but physiologically wired as another sex. It seems crazy but it happens regularly on an ordinary statistical distribution so it is simply part of human reproduction.

Think about that for a moment.

An example is the case of Riley Grant, who has been documented in the news. Riley’s body is biologically male. She has, I believe, a standard XY chromosome. She has a fully functioning male reproductive system. However, Riley’s brain didn’t develop as male during gestation and was mapped as female. We know from advances in neuroscience the past few decades that the differences between male and female brains are not insignificant – it influences everything from color perception to taste, scent, emotional reaction, empathy levels, rationality levels, pain tolerance, vocal inflection, and a host of many other factors. This is easy to see on an MRI – male and female brains respond differently to different stimuli. The largest study documenting the extent of the differences between male and female brains was done by Dr. Daniel Amen, who analyzed 26,000 people and found that the male brain has heightened activity in regions “associated with visual perception, tracking objects through space, and form recognition” and are 8% to 10% larger in mass size, while the female brain shows more overall activity, as well as increased blood flow in 112 out of 128 brain regions.

Riley’s parents realized this when they discovered her at 2 years old in the shower holding clippers against her penis saying, “It doesn’t go there.” She kept insisting she was a girl. Sure enough, a lot of medical tests later, that turned out to be the case. That means that, in this case, the physiological sex mapping of the brain is different from the biological sex of the body. Riley’s brain is wired as female despite having XY chromosomes. There is no question about it. It’s a fundamental, scientific, indisputable fact. It is not a mental disorder. She is not confused. Her brain is of the same structure as the typical woman. A century ago, she would have been written off as crazy or disturbed but our understanding of the interesting outcomes of biology now let us know that it’s a very real condition based upon demonstrable facts.

Sometimes, but not always, this condition is caused because a male fetus is immune to testosterone. When this happens, the testosterone released by the mother’s body during development doesn’t trigger the signal to map the brain as male, and a female mind is created, despite the fact the genetic instructions from the chromosomes is busy making the physical body male. The only way to remove the cognitive dissonance and prevent suicide, substance abuse, and a host of other coping mechanisms that lead inevitably to death and misery is sexual reassignment surgery, forcing the exterior body to line up with the brain. This, in effect, removes the constant exposures to said cognitive dissonance, and leads to far greater physical and mental health.

(This is not to say everyone who wants sexual reassignment surgery is legitimately a case of a brain and body mismatch. Some are simply mentally unhealthy and fixate on the notion of being transgender as a coping mechanism, only to regret the change later. A sociological manifestation of this phenomenon is the so-called “pretendbians” – men who insist they are women, dress in women’s clothes, and present as women, but then wish to retain their male biological parts whilst saying that they are lesbians who want to date other lesbians. This includes having penis-in-vagina sexual relationships. These lesbians, who by very definition are not interested in having penetrative sex with a biologically male body, are then accused of being “transphobic” and creating – this is the actual term – a “cotton ceiling” a play on words that borrows from the glass ceiling in female employment and the cotton construction of a typical pair of underwear. It’s a disturbingly misogynistic thing to believe as it implies that the the biological female lesbians owe their physical and emotional affection to someone who demands it and is incapable of meeting their needs. One author refers to these “pretendbians” as men engaged in a self-deluded form of “heterosexual kink”. In any event, they do tremendous damage to the political efforts of actual transgender people, like the Riley Grants of the world, who should be protected from employment discrimination, given access to mental health resources during transition, and supported in school during early childhood when beginning hormone treatment to rectify what is a very real biological condition. Men and women who fall into this faux form of transgenderism often display a litany of mental health and / or mood disorders.)

The Case of David Reimer

The flip case of Riley Grant is the now well-documented and studied case of David Reimer. He was born a boy in 1965, one of two identical twins. He was absolutely normal, XY karyotype, fully functioning reproduction system. His parents wanted him circumcised but the doctor botched the operation so badly that they decided to castrate him and transform his body into a woman’s through the use of estrogen injections when the parents realized he would never have a penis or be able to enjoy sexual relationships with women. The thinking at the time was the now-debunked idiocy that is known as the “blank slate” theory that humans are entirely a product of their environment and we can adapt to anything. The truth is, a lot of our personality is hardwired on a genetic level.

Despite putting David in “frilly dresses”, forcing him to play with female toys, calling him “Brenda”, and keeping the secret so that no one knew he was born a boy, David’s brain knew better. He kept insisting he was not a girl. He kept insisting he was not attracted to men, despite being told that, as a woman, he should be. By 13, he had grown suicidal as the cognitive dissonance between what people were telling him and he saw when he looked in a mirror and what his brain knew inherently grew too great. At 14 years old, he decided to live as a man, began taking testosterone injections, and undergoing cosmetic surgery. He married a woman and became stepfather to her kids. Only later did his parents confess what had happened to him, after he had finally decided he was willing to live as a man even if they didn’t accept it.

Nothing the doctors could do changed the fact that David was a male nor could they change his sexual orientation despite everyone around him insisting that he was a girl and was meant to date boys. His brain knew better. He was wired in a very specific way in the womb and no amount of elective cosmetic surgery or hormone treatment could change that.

Biological Sex Is Not the Same As Gender

What causes some confusion in the general public is the use of biological sex and gender as interchangeable terms. They do not refer to the same thing.

  • Biological Sex – Usually determined by karyotype. The brain, body, and reproductive system can be different sexes, in the case of legitimate transgender people, where the brain physiology resembles that of the opposite sex, or biological chimeras.
  • Gender – Mostly used for cultural behaviors such as dress, mannerisms, signs of deference, et cetera, that differentiate the sexes, gender itself is not entirely a social construct. As already mentioned, neuroscience research over the past few decades indicates through an overwhelming amount of evidence that gender is not a “blank slate” that is imparted entirely by civilization, but rather has some inherent characteristics that manifest regardless of upbringing or environment.

This is why some fringe activists can seriously say, “You can be a woman with a penis”, while most of the world will look at them like they have lost their mind. They are inherently using the term “woman” to refer to gender and not biological sex. This difference in vocabulary is responsible for virtually all conflicts between groups on issues in this arena. They do not realize they are using a phrase to refer to two separate things that are often, but not always, congruent.

The reality is the English language is woefully inadequate to address these biological, and in some cases, psychological, conditions. Unlike many ancient societies, we lack the requisite terms to make a differentiation. A person who is born male with a female brain and has sexual reassignment surgery can insist that she is a woman – and mentally, she is – but it is different than a fully formed, biological woman. And therein lies the trouble. Native American Indian tribes, Middle Eastern kingdoms … they had words to explain these things as they recognized reality a bit faster than we in the West have. It’s probably time to recognize that more than 99% of us are male or female, but in a world with so many billions of people, that 1% is a heck of a lot of folks who are something else. Trying to shove them into a binary system when the universe itself is not binary in this matter is a form of the mental model known as “greedy reductionism”. It stigmatizes them for a physical trait that is entirely benign and it damages us by causing us to ignore reality something that should be anathema to the rational thinker.

Follow Up Questions for Further Study

Here are some questions that I still need to address and consider:

  • In the case of legitimate transgender individuals with a brain and body that are not congruent, the best mental health outcome is to begin the correction and transition process to lower cognitive dissonance as early as possible, before the onset of puberty. However, if a mistake is made, the damage can be irreversible How should society, particularly medical doctors, proceed with this knowledge?
  • John Hopkins, one of the most respected medical institutions in the world, closed its gender reassignment center back in the 1980’s because the then-chairman of the psychiatric department, Paul McHugh, decided that he was helping mentally disturbed people mutilate their bodies instead of treating them to recognize reality. However, as stated previously, the vast advances in neuroscience now tell us that gender is almost entirely “innate and immutable” from the time we leave the womb. If you were in charge of John Hopkins, would you consider changing the institutions stance in light of the past twenty fives years of advances in understanding? Why or why not?
  • Does it change your opinion when you realize that even Iran, one of the most irrational and illogical societies on the planet, which often ignores scientific data, recognizes the condition and provides sexual reassignment surgery to individuals who are affected by the condition? Yes, they do it under the idiotic guise of thinking that gay men want to become women, which has nothing to do with transgenderism as a vast, vast majority of gay men are so-called cisgendered (their biological sex lines up with their gender identity) but the practical outcome is, someone could transition, even if for the wrong reasons.
  • Were you to have a transgender child, what would your course of action be?
  • What would you feel, emotionally, if you discovered you were not an XY or XX male or female? For example, if you were a male who was XYY? Would it matter to you?
  • Given that we now know humans are not made male and female, shouldn’t we come up with terms to describe the four other karyotypes that commonly manifest in births? If so, what should we call them?
  • Research the interesting phenomenon that discrimination against those who are not traditionally male or female are often treated with respect if, and only if, they “pass” and are attractive. In other words, the power of beauty capital is so enormous, it exceeds and overcompensates for inherent discrimination. We, as people, will forgive almost anything if a person is beautiful.
  • As uncovered by the economists behind the Freakonomics series, why do heterosexual men secretly consume enormous quantities of so-called “she-male” porn, involving beautiful women with both breasts and a penis, but gay men have virtually zero desire to see the same thing and are turned off by it? There is something here I’m missing that is the key to understanding a lot about biological drive. It’s too big, and odd, of a disparity.

For now, this topic needs to go back in the file cabinet and be revisited in future years until it is fully flushed out and concluded.


Essay Contents:

  1. Essay on the Introduction to the Process of Sex Determination
  2. Essay on the Chromosome Theory of Sex Determination
  3. Essay on Animals with Heterogametic Females
  4. Essay on the Process of Sex Determination in Human Beings
  5. Essay on Genic Balance Theory of Sex Determination in Drosophila
  6. Essay on Haplodiploidy and Sex Determination in Hymenoptera
  7. Essay on the Process of Sex Determination in Coenorhabditis Elegans
  8. Essay on Environmental Factors and Sex Determination

Essay # 1. Introduction to the Process of Sex Determination :

In nature a large number of diverse mechanisms exist for determination of sex in different species. The fruit fly Drosophila melanogaster and human beings are very important in the development of genetic concepts because in these two organisms, and in many others, individuals normally occur in one of two sex phenotypes, male or female.

In these species males produce male gametes, sperm, pollen or microspores while females produce female gametes namely, eggs, ovules or macrospores. In many species the two sexes are phenotypically indistinguishable except for the reproductive organs. Sex determination is aimed at identifying the factors responsible to make an organism a male or female or in some cases a hermaphrodite. So far the mechanism of sex determination has been related to the presence of sex chromosomes whose composition differs in male and female sexes.

However, in recent years sex determination has been differentiated from sex differentiation, and sex determination mechanism is explained more on the basis of the specific genes located on sex chromosomes and autosomes. Sex determination is recognized as a process in which signals are initiated for male or female developmental patterns.

During sex differentiation, events occur in definite pathways leading to the development of male and female phenotypes and secondary sexual characters. Significant progress has been made in understanding the mechanism of sex in human beings and other mammals and new genes have been identified.

Essay # 2. Chromosome Theory of Sex Determination :

Sex determination in higher animals is controlled by the action of one or more genes. The testis determining factor (TDF) gene is the dominant sex determining factor in human beings. Hemking a German biologist identified a particular nuclear structure throughout the spermatogenesis in some insects. He named it as “X-body” and showed that sperm differed by its presence or absence. The X body was later found to be a chromosome that determined sex. It was identified in several insects and is known as the sex or X chromosome.

Thus, the chromosome theory of sex determination states that female and male individuals differ in their chromosomes. Chromosomes can be differentiated into two types, autosomes and sex chromosomes. Sex chromosomes carry genes for sex. In some animals, females have one more chromosome than males, thus they have two X chromosomes and males have only one.

Females are therefore cytologically XX and males are XO, where ‘O’ denotes the absence of X chromosome. During meiosis in the female the 2X chromosome pairs and separates producing eggs that contain a single X chromosome. Thus all eggs are of the same type containing only one X chromosome.

During meiosis in the male, the single X chromosome moves independently of all the other chromosomes and is incorporated into half of the sperm, the other half do not have any X chromosome. Thus, two types of sperms are produced, one with X chromosome and the other without the X chromosome or designated as ‘O’.

When the sperm and eggs unite, two types of zygotes are produced XX that develop into females and XO that develop into males. Because both of these types are equal in number, the reproductive mechanism preserves a 1:1 ratio of males to females.

In many animals, including human beings, males and females have the same number of chromosomes. This numerical equality is due to the presence of a chromosome in the male called the ‘Y’ chromosome, which pairs with the X. During meiosis in the male, the X and Y-chromosomes separate from each other producing two types of sperm, one type with X chromosome and the other type having Y chromosome.

The frequencies of the two types are approximately equal. Females with XX chromosomes produce only one type of eggs, all with X chromosome. In random fertilization, approximately half of the zygotes are with XX chromosomes and the other half with XY chromosomes leading to a sex ratio of 1:1.This mechanism is called XX – XY type of sex determination.

The XY mechanism is more prevalent than the XO mechanism. The XY type is considered characteristic in higher animals and occurs in some plants. This mechanism is operative in Drosophila melanogaster and human beings. Both species exhibit the same pattern of transmission of X and Y chromosomes in normal individuals in – natural populations. In human beings, the X chromosome is considerably longer than the Y chromosome.

The total complement of human chromosomes includes 44 autosomes: XX in the female and XY in the male. Eggs produced by the female in oogenesis have a complement of 22 autosomes plus an X chromosome. Sperm from the male have the same autosomal number and either an X or a Y chromosome. Eggs fertilized with sperm containing a Y chromosome result in zygotes that develop into males those fertilized with sperm containing an X chromosome develop into females.

In animals with XX-XY mechanism of sex determination, females (XX) produce gametes that have the same chromosome composition (one X plus one set of autosomes). These females are homogametic sex as all the gametes are the same. The males of these animals are heterogametic as they produce two types of gametes, one half containing one X chromosome plus one set of autosomal chromosomes and the other one half contain one Y chromosome plus one set of autosomes.

Essay # 3. Animals with Heterogametic Females:

In many birds, moths and some fish, the sex determination mechanism is identical to the XX-XY mechanism but the females are heterogametic (ZW) and males are homogametic (ZZ). This mechanism of sex determination is called ZZ-ZW.

In this mechanism the relationship between sex chromosomes and sex phenotypes is reversed. In birds the chromosome composition of the egg determines the sex of the offspring, whereas in humans and fruit flies, the chromosome composition of the sperm determines the sex of the offspring.

Essay # 4. Process of Sex Determination in Human Beings:

In human beings, sex is determined by the number of X chromosomes or by the presence or absence of the Y chromosome. In human beings and other placental mammals, maleness is due to a dominant effect of the Y chromosome. The dominant effect of the Y chromosome is manifested early in development when it directs the primordial gonads to differentiate into testes.

Once the testes are formed, they secrete testosterone that stimulates the development of male secondary sexual characteristics. Testis determining factor (TDF) is the product of a gene called SRY (Sex determining Region of Y), which is located in the short arm of the Y chromosome of the mouse. SRY was discovered in unusual individuals whose sex was not consistent with their chromosome constitution – males with XX chromosomes and females with XY chromosomes.

Some of the XX males carried a small piece of the Y chromosome inserted into one of the X chromosomes. It is evident that this small piece carried genes for maleness. Some of the XY females carried an incomplete Y chromosome. The part of the Y chromosome that was missing corresponded to the piece that was present in the XX males.

Its absence in the XY females prevented them from developing testes. These observations show that a particular segment of the Y chromosome was required for the development of the male. Further studies showed that the SRY gene is located in this male determining segment. Like that of the human SRY gene is present in the Y chromosome of the mouse and it specifies male development (Fig. 5).

After the formation of the testes, testosterone secretion initiates the development of male sexual characteristics. The hormone testosterone binds to receptors of several types of cells. This binding leads to the formation of a hormone – receptor complex that transmits signals to the cell instructing how to differentiate.

The combined differentiation of many types of cells leads to the development of male characteristic like beard, heavy musculature and deep voice. Failure of the testosterone signaling system leads to nonappearance of the male characters and the individual develops into a female. One of the reasons for failure is an inability to make the testosterone receptor (Fig. 6).

Individuals with XY chromosomal composition having this biochemical deficiency first develop into males. In such males, although testis is formed and testosterone secreted, it has no effect because it cannot reach the target cell to transmit the developmental signal. Individuals lacking the testosterone receptor therefore can change sexes during embryological development and acquire female sexual characteristics.

However, such individuals do not develop ovaries and remain sterile. This syndrome known as testicular feminization is due to a mutation in an X-linked gene, tfm that codes for the testosterone receptor. The tfm mutation is transmitted from mothers to sons who are actually phenotypically female in a typical X- linked manner.

Master Regulatory Gene:

In human beings irregular sex chromosome constitutions occur occasionally. Any number of X chromosomes (XXX or XXXX), in the absence of a Y chromosome give rise to a female. For maleness, the presence of a Y chromosome is essential and even if several X chromosomes are present (XXXXY), the presence of a single Y chromosome leads to maleness.

The Y chromosome induces the development of the undifferentiated gonad medulla into testis, whereas an XX chromosomal set induces the undifferentiated gonadal cortex to develop into ovaries. The gene on the Y chromosome that induces the development of testes is called as Testis Determining Factor (TDF). It has been isolated, characterized and found to encode a protein that regulates the expression of other genes.

Thus, the TDF gene is the master regulator gene that triggers the expression of large number of genes that produce male sex phenotype. In the absence of TDF gene, the genes that produce femaleness predominate and express to produce a female phenotype. The TDF exerts a very dominant effect on development of the sex phenotype.

Essay # 5. Genic Balance Theory of Sex Determination in Drosophila:

In Drosophila investigations by C.B. Bridges have shown that X chromosomes contain female determining genes and male determining genes are located on the autosomes and many chromosome segments are involved. The genie balance theory of sex determination in Drosophila explains the mechanism involved in sex determination in this fly.

The Y chromosome in Drosophila does not play any role in sex determination. Sex in this animal is determined by the ratio of X chromosomes to autosomes. Normal diploid insects have a pair of sex chromosomes, either XX or XY, and three pairs of autosomes. These are denoted by AA, each A representing one set of haploid autosomes. Flies with abnormal number of autosomes can be produced by genetic manipulation as shown in Table 1.

Whenever the ratio of X chromosomes to autosomes is 1.0 or above, the sex of the fly is female, and whenever it is 0.5 or less, the fly is male. If the ratio is between 0.5 and 1.0, it is an intersex with both male and female characters. In all these phenotypes, Y chromosome has no role to play but it is required for the fertility of the male. In Drosophila sex determination mechanism, an X-linked gene called Sex lethal (Sxl) plays an important role (Fig. 7).

A number of X linked genes sets the level of Sxl activity in a zygote. If the ratio between X chromosomes and autosomes is 1.0 or above, the Sxl gene becomes activated and the zygote develops into a female. If the ratio is 0.5 or less, the Sxl gene is inactivated and the zygote develops into a male. A ratio between 0.5 and 1.0 leads to mixing of signals and the zygote develops into an intersex with a mixing of male and female characters.

The sex ratio of X chromosomes to autosomes and the phenotype of Drosophila determination pathway in Drosophila has three components:

(i) A system to ascertain the X : A ratio in the early embryo,

(ii) A system to convert this ratio into a developmental signal, and

(iii) A system to respond to this signal by producing either male or female structures.

The system to ascertain the X : A ratio involves interactions between maternally synthesized proteins that have been deposited in the eggs cytoplasm and embryologically synthesized proteins that are coded by several X-linked genes. These latter proteins are twice as abundant in XX embryos as in XY embryos and therefore provide a means for counting the number of X chromosomes present.

Because the genes that encode these proteins effect the numerator of the X : A ratio, they are called numerator elements. Other genes located on the autosomes affect the denominator of X : A ratio and are therefore called as denominator elements. These encode proteins that antagonize the products of numerator elements (Fig. 8).

The system for ascertainment of the X : A ratio in Drosophila is therefore based on antagonism between X-linked (numerator) and autosomal (denominator) gene products. Once the X : A ratio is ascertained, it is converted into a molecular signal that controls expression of the X-linked sex lethal gene (Sxl), the master regulator of the sex determination pathway.

Early in development, this signal activates transcription of the Sxl gene from PE’ the gene’s ‘early’ promoter, but only in XX embryos. The early transcripts from this promoter are processed and translated to produce functional sex-lethal proteins, denoted Sxl. After only a few cell divisions, transcription from the PE promoter is replaced by transcription from another promoter, PM.

The so called maintenance promoter of the Sxl gene. Interestingly, transcription from the PM promoter is also initiated in XY embryo. However, the transcripts from PM are correctly processed only if Sxl protein is present. Consequently, in XY embryos, where this protein is not synthesized, the Sxl transcripts are alternately spliced to include an exon with a stop codon, and when these alternately spliced transcripts are translated, they generate a short polypeptide without regulatory function.

Thus, alternate splicing of the Sxl transcripts in XY embryos does not lead to the production of functional Sxl protein and in the absence of this protein, these embryos develop as males. In XX embryos, where Sxl protein was initially made in response to X : A signal, Sxl transcripts from the PM promoter are spliced to produce more Sxl proteins.

In XX embryos, this protein is therefore, a positive regulator of its own synthesis forming a feedback mechanism that maintains the expression of the Sxl proteins in XX embryos and prevents its expression in XY embryos. The Sxl protein also regulates the splicing of transcription from another gene in the sex determination pathways, transformers (tra). These transcripts can be processed in two different ways.

In chromosomal males, where the Sxl protein is absent, the splicing apparatus always leaves a stop codon in the second exon of the tra RNA. Thus, when spliced tra RNA is translated, it generates a truncated polypeptide. In females, where the Sxl protein is present, this premature stop codon is removed by alternate splicing in at least some of the transcripts. Thus, when they are translated, some functional transformer protein tra is produced. The Sxl protein therefore allows the synthesis of functional tra protein in XX embryos but not in XY embryos (Fig. 9).

The tra protein also turns out to be a regulator of RNA processing. Along with tra 2, a protein encoded by the transformer 2 (tra 2) gene, it encodes the expression of double sex (dsx) an autosomal gene that can produce two different proteins -through alternate splicing of its RNA. In XX embryos, where the tra protein is present, dsx transcripts are processed to encode a DSX protein that represses the genes required for male development.

Therefore, such embryos develop into females. In XY embryos, where the TRA protein is absent, dsx transcripts are processes to encode a DSX protein that represses the gene required for female development. Consequently, such embryos develop into males. The dsx gene is therefore, the switch point at which a male or female developmental pathway is chosen. From this point, different sets of genes are specifically expressed in males and females to bring about sexual differentiation.

Essay # 6. Haplodiploidy and Sex Determination in Hymenoptera:

In the order hymenoptera including bees, wasps, ants and sand flies, males develop parthenogenetically from unfertilized eggs and have a haploid chromosomal number (in honey bee drone, there are 16 chromosomes). The queen honeybee and workers develop from fertilized eggs and carry the diploid number of 32 chromosomes. Because the normal males are haploid and normal females are diploid, this mechanism is known as haplodiploidy.

The hemizygous, hortiozygous and heterozygous status of certain chromosome segments controls sex determination. Female determination depends on heterozygosity for part of a chromosome. If different forms of this segment of chromosome involved are designated Xa, Xb and Xc, then individuals of chromosome make up XaXb, XaXc and XbXc are all females.

Hemizygous individuals Xa, Xb, or Xc cannot be heterozygous and are therefore male. Genetic manipulations to produce homozygous diploid males showed that sex determination depends on the genetic composition of this region and not on diploidy versus haploidy (XaXa, XbXb, or XcXc).

Mosaics and Gynandromorphs:

Abnormal chromosomal behaviour in insects produces sexual mosaics or gynandromorphs. In these forms some parts of the animal are male and others are female. When such abnormal chromosomal transmission involves autosomes lodging genes that control easily recognized phenotypes, individuals may also be produced that are mosaic for phenotypes unrelated to sex phenotype. Some gynandromorphs in Drosophyla are bilateral intersexes (Fig. 10) with male color pattern body shape and sex comb on one half of the body and female characteristics on the other half of the body. Both male and female gonads and genitalia are present.

The reason for bilateral gynandromorphism is irregularity in mitosis at the first cleavage of the zygote (Fig. 11). A chromosome lags behind in division and does not arrive at the pole in tine to be included in the newly formed daughter nucleus. When one of the X chromosomes of an XX female zygote lags behind in the spindle, one daughter nucleus receives only one X chromosome, while the other receives two X chromosomes resulting in a mosaic body pattern.

One nucleus in the two nuclei stage would be XO male. If the cleavage plane is so oriented that one daughter nucleus goes towards the right, that part will give rise to all cells that make up the right half of the adult body and the other half gives rise to the left half. If the loss of chromosome occurs at a later stage in cell division, smaller parts of the adult body would be male.

Position and size of the mosaic sector are determined by the place and time of the division abnormality.

Essay # 7. Process of Sex Determination in Coenorhabditis Elegans:

Coenorhabditis elegans is a nematode hermaphrodite species having two X chromosomes and five pairs of autosomes. Occasionally animals with a single X chromosome and five pairs of autosomes are produced by meiotic non disjunction. These animals are males capable of producing sperms but not eggs. Hermaphrodites are females in their vegetative parts (soma) but mixed in their genetic composition.

The somatic sex determination pathway in C.elgans involves atleast 10 different genes. The tra-1 and tra-2 gene products are required for normal hermaphrodite development and that the her-1 gene product is needed for normal male development. The fem gene products fem-1, fem-2, fem-3 are also needed for normal male development. The gene her-1, encodes a secreted protein that is likely to be a signaling molecule.

The next gene, tra-2i encodes a membrane bound protein, which may function as a receptor for the her-1 signalling protein. The products of the fem genes are cytoplasmic proteins that may transduce the her-1 signal and the last gene in the pathway, tra-1 encodes a zinc finger type transcription factor, which may regulate the gene involved in sexual differentiation (Fig. 12).

In Coenorhabditis elegans the sex determination pathway involves a series of negative regulators of gene expression. In XO animals the secreted her-1 gene product apparently interacts with the tra-2 gene product, causing it to become inactive. This interaction allows the three fem gene products to be activated and they collectively inactivate the tra-1 gene product that is a positive regulator of female differentiation. Because the animal cannot develop as a hermaphrodite without active tra-1 protein, it develops into a male.

In XX animals, the her-1 protein is not formed, therefore its putative receptor, the tra-2 protein remains active. Active tra-2 protein causes the fem gene products to be inactivated, which in turn allows the tra-1 protein to stimulate differentiation of the female. The animal therefore develops into a hermaphrodite.

Sexual development in Caenorhabditis fundamentally depends on the X : A ratio, just as it does in Drosophila. The X : A ratio is somehow converted into a. molecular signal that controls sexual differentiation. The signal from the X : A ratio is directed into the sex determination and dosage compensation pathways through a short pathway involving at least four genes. One of these genes, xol -I is required in males but not in hermaphrodites. Three other genes, Sdc-1, Sdc-2 and Sdc-3 are negatively regulated by Xol-i. These Sdc genes are needed in hermaphrodites but not in males.

Development of animals is sensitive to an imbalance in the number of genes. Normally each gene is present in two copies. Departures from this condition, either up or down can produce abnormal phenotypes and sometimes even death. It is therefore, puzzling that many species have a sex determination system based on females with two X chromosomes and males with only one X chromosome.

Normal females have IX chromosomes when male has IX chromosome. This is an unique situation as the number of chromosomes is same in males and females. Such disparities or differences create a “genetic dosage” problem between males and females for all the X-linked genes.

Some females have two copies of X-chromosome and males only one. Therefore, there is potential for females to produce twice as much of each gene product for all the X-linked genes. For compensating this dosage problem, it is proposed that one of the X-chromosome becomes heterochromatin in the case of the female, so that dosage of genetic information expressed in both females and males is equal.

Dosage Compensation in Drosophila:

In Drosophila dosage compensation of X-linked genes is achieved by an increase in the activity of these genes in males. This phenomenon, called “hyperactivation” involves complex of different proteins that binds to many sites on the X-chromosome in males and triggers a doubling of gene activity. When this protein complex does not bind, as in the case of females, hyperactivation of X-linked genes does not occur. In this way total X-linked gene activity in males and females is approximately equal (Fig. 13).

Dosage Compensation in Humans:

In human beings dosage compensation of X-linked genes is achieved by the “inactivation” of one of the females X-chromosomes. This mechanism was first proposed by Mary Lyon in 1961. The chromosome to be inactivated is chosen at random. Once chosen it remains inactivated in all the descendants of that cell. In human embryos sex chromatin bodies have been observed by the 16 th day of gestation. Some human traits are influenced by both X chromosomes during the first 16 days. Later only one X chromosome is functional.

Thus, the female is a mosaic with some parts having the alternate allele expressed. X chromosome inactivation occurs only when at least two X chromosomes are present. When a number of X chromosomes are present in the same nucleus, all but one are inactivated. The number of sex chromatin bodies present after inactivation is one less than the number of X chromosomes present in the original cell.

Dosage Compensation in Caenorhabditis Elegans:

In C.elegans dosage compensation involves the partial repression of X-linked genes in the somatic cells of hermaphrotites. In C.elegans dosage compensation is achieved by “hypoactivating” the two X chromosomes in XX hermaphrodites.

Essay # 8. Environmental Factors and Sex Determination:

The environmental factors determine whether an individual develops into a male and female. They live as parasites in the reproductive tract of the well developed and bigger female. In male all organs except the reproductive system are degenerate. During reproduction, the female releases eggs into the seawater. The eggs hatch out to release young worms. Some of the young worms reach the proboscis of the female and become males.

They reach the female reproductive tract and lie as permanent parasites on the female. The young worms, which fail to reach a female, develop to become females. Genetic determiners for both the sexes are present in all young worms. It has been observed that the young worms become attracted towards the extracts of the female proboscis and become males.

In some reptiles, temperature plays an important role in determining the sex. In the turtle Chrysema picta incubation of eggs prior to hatching at high temperature leads to the development of females. However, in the lizard

Agama high incubation temperature leads to male progeny.

Although the segregation of specific sex determining genes and chromosomes is responsible for sex in most animals, the genetic potential for both maleness and femaleness exists in every zygote and some specific factor in the environment triggers the expression of maleness or femaleness producing genes resulting in the production of male phenotype or female phenotype.


Genetics of Male Infertility

Klinefelter syndrome

The most common sex chromosome aneuploidy in humans is the KS, which may manifest with different chromosomal constitution: 47, XXY or mosaic 46, XY/47, XXY, or higher-grade sex chromosomal aneuploidy, that is, 48, XXXY, 49, XXXXY, etc. Although its incidence is high (1:660 in live births and 1:300 in spontaneous abortion), the disease is often undiagnosed due to the large phenotypic variability of the disease. The only constant finding in affected individuals is the presence of small, firm testes due to hyalinization of seminiferous tubules. Azoospermia is present in over 90% of patients, while the remaining semen phenotype can be crypto/severe oligozoospermia (mainly in mosaic cases of KS). After puberty, the large majority of these patients present signs of androgen deficiency these signs range from hypogonadism with gynecomastia and eunuchoid body proportions to variable levels of undervirilization ( Aksglaede and Juul, 2013 ). In addition to reproductive/sexual dysfunctions, KS patients present higher morbidity for a series of diseases such as metabolic syndrome, autoimmune diseases, venous thromboembolism, and cognitive/psychiatric disturbances ( Calogero et al., 2017 ).


What are Autosomes

Non-sex chromosomes which determine the trait of an organism is identified as autosomes. They are also known as somatic chromosomes since they determine the somatic characters of an individual. A genome mainly consists of autosomes. For example, human body contains 46 chromosomes within its genome and 44 chromosomes of them are autosomes. Autosomes exist as homologous pairs and 22 autosome pairs can be identified in the human genome.

Both autosomal chromosomes contain the same genes, which are arranged in the same order. But an autosomal chromosome pair differs from other autosomal chromosome pairs within the same genome. These pairs are labeled from 1 to 22, according to the base pair sizes contained in each chromosome.

Autosomes also participate in sex determination. SOX9 gene is an autosomal gene on chromosome 17. It activates the function of TDF factor which is encoded by Y chromosome. TDF factor is critical in male sex determination. Hence, a mutation of SOX9 causes the development of Y chromosome, resulting in a female.

Autosomal genetic disorders occur due to either the non-disjunction in parent chromosomes (Aneuploidy) during gametogenesis or the Mendelian inheritance of deleterious alleles. An example for aneuploidy is Dawn’s Syndrome, which possesses three copies of chromosome 21 per cell. Disorders with Mendelian inheritance can either be dominant or recessive (Ex: Sickle cell anemia).

Figure 1: Human male karyotype


Sex chromosomes

Sex Chromosomes Definition
Sex chromosomes are chromosomes that determine whether the individual is male or female.

Sex chromosomes are chromosomes that contain information that is used to determine the gender of an organism. The expression of a sex chromosomeon an allele can either be X or Y. Organisms with sex chromosomes XY are male while organisms with sex chromosomes XX are female.

Sex Chromosomes and Sex-Linked Inheritance
Study Sex Chromosomes and Sex-Linked Traits
Sex Determination in Humans .

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Sex Determination » .

are X and Y. Females have two X chromosomes, and males have an X and a Y.

[edit]
The Y chromosome does not undergo recombination, making it particularly prone to the fixation of deleterious mutations via hitchhiking. This has been proposed as an explanation as to why there are so few functional genes on the Y chromosome.[11]
Gene surfing[edit] .

The chromosomes that determine the sex of an organism. In humans, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Chromosome that determines the gender (sex) of the individual.

The pair of chromosomes responsible for determining the sex of an individual.
sex-linked genes .

Chromosomes that determine the sex of an individual. There are two sex chromosome systems (1) the XX /XY system, where XY individuals are male, and XX individuals are female (the usual system, for example, in mammals and butterflies) and (2) the WW/WZ system where WW individuals are male, .

are the ones that determine your gender. These are X and Y (XX in females, XY in males).

appear as XX or YY. When the sex cells form these pairs separate. Females carry XX and male carry XY. The normal egg cells produced by a human ovary have X cells while sperm carry X in half and Y in the other half. It's the sperm which decides what the babies' sex is.

that are fully differentiated the way we know them with an X and a Y chromosomes.

60
Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes (see the table). Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell.

are one of the 23 pairs of human chromosomes. The X chromosome spans more than 153 million base pairs (the building material of DNA) and represents about 5 percent of the total DNA in cells.

of XY.
Which of a women's grandparents could not be the source of any of the genes on either of her X-chromosomes? A. Mother's Father.
B. Father's Mother.

determines an individual's sex.

, X and Y.
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If you know of any terms that have been omitted from this glossary that you feel would be useful to include, please send details to the Editorial Office at GenScript.

In summary, we've learned that

and inherit the genes of that parent only assimilation in the nitrogen cycle, when living organisms take up nitrogen atom the smallest unit or the basic building blocks of matter which make up all objects, made up of equal number of protons and electrons autosomes chromosomes that are not

Genome -- all of the genes carried by a single gamete the DNA content of an individual, which includes all 44 autosomes, 2

, and the mitochondrial DNA. Genotype -- genetic constitution of an organism. Germ cell -- a sex cell or gamete (egg or spermatozoan).

Humans have 23 pairs in all: 44 autosomes and two

. Each parent contributes one to each pair, so children get half of their chromosomes from their mother and half from their father.
Clone - A group of genetically identical organisms.

An individual with the appropriate number of chromosomes for their species is called euploid in humans, euploidy corresponds to 22 pairs of autosomes and one pair of

Typically in mammals, the gender of an organism is determined by the

. In the case of humans, this happens to be the X and the Y chromosomes. So as you may recall, if you are XX, you are female. If you are XY, you are male.

that is always expressed, even if only one copy is present. The chance of passing the gene to offspring is 50% for each pregnancy.
See also: autosome, dominant, gene (ORNL)
Autosome
A chromosome not involved in sex determination.

. Sex is therefore determined by fertilization. If an egg becomes fertilized, it will develop into a male. Unfertilized eggs develop into females.

reverse sex and hatch as females.

A human cell contains 23 pairs of homologous chromosomes: 22 of them are homologous non-

are 2 X's in males the X and Y chromosomes.
Compare: sister chromatids.
See also: allele, dominant, recessive.

are different from each other. In mammals, most other vertebrates and most insects, males are the heterogametic sex (XY), whereas in birds, lepidopterans, and some fish it is females (WZ).

Some genes are part of the

and so are inherited with them. Usually it is the X chromosome that is considered in which case the female will have two alleles, the male will only have one.
The genetic condition of haemophilia is carried on the X chromosome.

usually refers to X-linked genes
Source: Jenkins, John B. 1990. Human Genetics, 2nd Edition. New York: Harper & Row
.

We have two chromosomes, the X and the Y which are the sex determining chromosomes and are therefore called the

. However, the X and Y chromosomes have genes for traits other than sex. The genes on the X form one linkage group and the genes on the Y form another linkage group.

The sex chromosome that is present in both sexes: singly in males and doubly in females human females normally have two X chromosomes.One of the

Heterogametic sex: The sex, which has the two different

(XY). Human and Drosophila males are the heterogametic sex, whereas, in birds, moths, some fish and amphibians, females are the heterogametic sex (ZW).

10) The X and Y chromosomes, while different behave as homologous chromosomes during meiosis. They link up at the pseudoautosomal regions (PAR). X and Y are called the

to distinguish them from the other 22 autosomal chromosomes.

Autosome: A chromosome not involved in sex determination. The diploid human genome consists of 46 chromosomes, 22 pairs of autosomes, and 1 pair of

(awtuh-some) [Gk. autos, self + soma, body]
A chromosome that is not directly involved in determining sex, as opposed to the

Karyotype. The number of chromosomes present in a given genome and the morphologic form that they assume (banding patterns, etc.) under microscopic examination. The laboratory mouse has 20 pairs of chromosomes (19 autosomal pairs and the X and Y


An Introduction to Genetic Analysis. 7th edition.

Most animals and many plants show sexual dimorphism in other words, an individual can be either male or female. In most of these cases, sex is determined by special sex chromosomes. In these organisms, there are two categories of chromosomes, sex chromosomes and autosomes (the chromosomes other than the sex chromosomes). The rules of inheritance considered so far, with the use of Mendel’s analysis as an example, are the rules of autosomes. Most of the chromosomes in a genome are autosomes. The sex chromosomes are fewer in number, and, generally in diploid organisms, there is just one pair.

Let us look at the human situation as an example. Human body cells have 46 chromosomes: 22 homologous pairs of autosomes plus 2 sex chromosomes. In females, there is a pair of identical sex chromosomes called the X chromosomes. In males, there is a nonidentical pair, consisting of one X and one Y. The Y chromosome is considerably shorter than the X. At meiosis in females, the two X chromosomes pair and segregate like autosomes so that each egg receives one X chromosome. Hence the female is said to be the homogametic sex. At meiosis in males, the X and the Y pair over a short region, which ensures that the X and Y separate so that half the sperm cells receive X and the other half receive Y. Therefore the male is called the heterogametic sex.

The fruit fly Drosophila melanogaster has been one of the most important research organisms in genetics its short, simple life cycle contributes to its usefulness in this regard (Figure 2-11 ). Fruit flies also have XX females and XY males. However, the mechanism of sex determination in Drosophila differs from that in mammals. In Drosophila, the number of X chromosomes determines sex: two X’s result in a female and one X results in a male. In mammals, the presence of the Y determines maleness and the absence of a Y determines femaleness. This difference is demonstrated by the sexes of the abnormal chromosome types XXY and XO, as shown in Table 2-3 . However, we postpone a full discussion of this topic until Chapter 23 .

Figure 2-11

Life cycle of Drosophila melanogaster, the common fruit fly.

Table 2-3

Chromosomal Determination of Sex in Drosophila and Humans.

Vascular plants show a variety of sexual arrangements. Dioecious species are the ones showing animal-like sexual dimorphism, with female plants bearing flowers containing only ovaries and male plants bearing flowers containing only anthers (Figure 2-12 ). Some, but not all, dioecious plants have a nonidentical pair of chromosomes associated with (and almost certainly determining) the sex of the plant. Of the species with nonidentical sex chromosomes, a large proportion have an XY system. For example, the dioecious plant Melandrium album has 22 chromosomes per cell: 20 autosomes plus 2 sex chromosomes, with XX females and XY males. Other dioecious plants have no visibly different pair of chromosomes they may still have sex chromosomes but not visibly distinguishable types.

Figure 2-12

Two dioecious plant species: (a) Osmaronia dioica (b) Aruncus dioicus. (Part a, Leslie Bohm part b, Anthony Griffiths.)

Cytogeneticists have divided the X and Y chromosomes of some species into homologous and nonhomologous regions. The latter are called differential regions (Figure 2-13 ). These differential regions contain genes that have no counterparts on the other sex chromosome. Genes in the differential regions are said to be hemizygous (“half zygous”) in males. Genes in the differential region of the X show an inheritance pattern called X linkage those in the differential region of the Y show Y linkage. Genes in the homologous region show what might be called X-and-Y linkage. In general, genes on sex chromosomes are said to show sex linkage.

Figure 2-13

Differential and pairing regions of sex chromosomes of humans and of the plant Melandrium album. The regions were located by observing where the chromosomes paired up in meiosis and where they did not.

The genes on the differential regions of the sex chromosomes show patterns of inheritance related to sex. The inheritance patterns of genes on the autosomes produce male and female progeny in the same phenotypic proportions, as typified by Mendel’s data (for example, both sexes might show a 3:1 ratio). However, crosses following the inheritance of genes on the sex chromosomes often show male and female progeny with different phenotypic ratios. In fact, for studies of genes of unknown chromosomal location, this pattern is a diagnostic of location on the sex chromosomes. Let’s look at an example from Drosophila. The wild-type eye color of Drosophila is dull red, but pure lines with white eyes are available (Figure 2-14 ). This phenotypic difference is determined by two alleles of a gene located on the differential region of the X chromosome. When white-eyed males are crossed with red-eyed females, all the F1 progeny have red eyes, showing that the allele for white is recessive. Crossing the red-eyed F1 males and females produces a 3:1 F2 ratio of red-eyed to white-eyed flies, but all the white-eyed flies are males. This inheritance pattern is explained by the alleles being located on the differential region of the X chromosome in other words, by X-linkage. The genotypes are shown in Figure 2-15 . The reciprocal cross gives a different result. A reciprocal cross between white-eyed females and red-eyed males gives an F1 in which all the females are red eyed, but all the males are white eyed. The F2 consists of one-half red-eyed and one-half white-eyed flies of both sexes. Hence in sex linkage, we see examples not only of different ratios in different sexes, but also of differences between reciprocal crosses.

Figure 2-14

Red-eyed and white-eyed Drosophila. (Carolina Biological Supply.)

Figure 2-15

Explanation of the different results from reciprocal crosses between red-eyed (red) and white-eyed (white) Drosophila. (In Drosophila and many other experimental systems, a superscript plus sign is used to designate the normal, or wild-type allele. (more. )

In Drosophila, eye color has nothing to do with sex determination, so we see that genes on the sex chromosomes are not necessarily related to sexual function. The same is true in humans, for whom pedigree analysis has revealed many X-linked genes, of which few could be construed as being connected to sexual function.

MESSAGE

Sex-linked inheritance regularly shows different phenotypic ratios in the two sexes of progeny, as well as different ratios in reciprocal crosses.

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