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Biologists Debunk the "Rotting Y Chromosome" Theory
An anonymous reader writes "Biologists have previously predicted that that the male sex-determining Y chromosome, which once carried around 800 genes, like the X, has lost hundreds of them over the past 300 million years, will mutate itself out of existence, leading to the eventual extinction of men. However, researchers of a study published in the latest issue of Nature found evidence to suggest that the Y chromosome will not shed any more of the 19 ancestral genes that it is left with."
Not like I love the Nature Publishing Group (NPG) very much, but let's link to the source to help give the original authors credit. (Which, as far as I can tell, the medical daily article doesn't even do!)
Here is a link to the original paper
For those who aren't molecular biologists or geneticists, here is a link to the Nature news article on the scientific paper
-Ryan
AUWYHSTOT (Acronyms are Useless When You Have to Spell Them Out Too)
No, the XX/XY system of sex determination is just one of the many types used in nature, mostly by mammals. Some animals use the XX(female)/X0(male) system, like ants and bees, while reptiles, birds and some other use the ZZ(male)/ZW(female) system. As you can see, in birds and reptiles (not crocodilians or turtles those have a temperature based sex determination mechanism) the ZZ chromosome configuration (the default) is male, while the ZW configuration causes female development. However, back to the disappearing Y chromosome, it is the fastest mutating chromosome in the human genome (and in all mammalian genomes) because it does not recombine with an analogue chromosome, the way two X chromosomes would. However, just because genes are "lost" does not mean it is shrinking, and research stated here shows it to be the case.
It's worth noting that conditions apart from standard XX female and XY male do occur in humans:
Turner syndrome: usually, single X chromosome, no second X or Y. Creates females who are almost always infertile with varying physical problems. Incidence is around 1 in 2000 to 1 in 5000 (phenotypic) females.
Triple X syndrome: XXX chromosomes. Makes females with essentially no physical differences from XX females (including reproductively). Incidence: 1 in 1000 females.
Klinefelter's Syndrome: XXY chromosomes. Produces sometimes-infertile males, sometimes with developmental problems. 1 in between 500 and 1000 males affected.
XYY Syndrome: XYY chromosomes. Almost no physical differences with XY males (slightly taller). 1 in 1000 males.
XX Male Syndrome: XX chromosomes. Produces always-infertile males who usually appear to be XY males. 4 or 5 in 100,000 people.
Swyer Syndrome: XY chromosomes. Produces females without developed gonads, though a developed uterus may be able to carry another person's embryo.
The above is only a partial list. There are quite a few related conditions that fall under the general heading of "Intersex" (sometimes you see the acronym LGBTI; that's the I). They vary widely from producing (some type of) hermaphrodites to causing a large number of non-standard sex characteristics. From the article,
According to Fausto-Sterling's definition of intersex, on the other hand, 1.7 percent of human births are intersex.
and
Between 0.1% and 0.2% of live births are ambiguous enough to become the subject of specialist medical attention, including surgery to disguise their sexual ambiguity.
To give a very approximate comparison (these numbers vary a lot by region, time period, and definition), around 1% of the population is bisexual, and around 5% is gay. It's perhaps even more difficult to get an accurate transgender incidence number; I've seen between 0.2% and 0.003%. Those who get sex reassignment surgery are in the minority. (There's a lot more to gender than the type of gonads you have, and female-to-male surgery isn't terribly effective.)
Crossing over is normally a beneficial practice for chromosomes, and is a key advantage for sexual reproduction. See Muller's ratchet for the case of deleterious effects piling up in organisms who do not use recombination to shuffle around their genes. Therefore, it generally makes sense for chromosomes to pair up, so that at each generation, offspring get a mix of ancestral genes rather than a perfect (aside from mutation, of course) clone of a chromosome from parent's set. Being able to swap homologous genes dramatically increases the diversity of offspring. The X and Y pair is an exception because it would be a problem for recombination to occur. The genes on Y are supposed to be unique to males- if some regions of Y containing them recombined with X, those male-development specific genes could be traded away in exchange for nothing, likely leading to sterile offspring. Therefore, it has been to the advantage of XY-determination organisms to have X and Y as different as possible so that there is basically no recombination. The drawback is that Y doesn't get to recombine with anything (X can still crossover with another X), so a son's Y is essentially his father's Y and grandfather's Y, though with whatever errors have accumulated. This has led to a pruning of genes on Y over time- it appears from this paper that this deletion will not necessarily go to nothing, and that a "minimal Y" may be stable for many millions of years. It does however underline that the X/Y pair is a special case, that the endpoint of the asymmetric relationship that they have is for one chromosome to dwindle to a single purpose. The Y chromosome has 19 genes: X has around 2000.
"FDA staff reviewers expressed concern about the number of patients who were left out of the study because they died."
Once again with the subject of genetics on Slashdot, we have a shocking level of confident ignorance on display (aided and abetted by the equally clueless moderators).
Please, evolution is not synonymous with natural selection. If all you know about genetics is what you learned in Biology 101, perhaps supplemented by a Dawkins book, you're missing out on most of the picture.
The degeneration of the Y chromosome was made possible by the lack of recombination along most of its length (Muller's ratchet/Hill-Robertson effect), which allowed the combined effects of mutation (including deletions) and genetic drift (which is much stronger on the Y due to there being 1/4 the number of Y chromosomes in a population than a given autosome) to very slowly truncate it. There's really no need to invent post-hoc selective stories to explain this; it's all pretty basic stuff.
Of course, you are correct that this doesn't mean that males would (or could) go extinct if the Y somehow did disappear. No competent scientist would ever claim this; most likely the sex-determining genes would move to other chromosomes.
Summary of this story in Nature
The origins of genome complexity