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Republish our articles for free, online or in print, under Creative Commons licence. The Y chromosome, that little chain of genes that determines the sex of humans, is not as tough as you might think. So will it one day completely disappear? And what happens to the human race if it does? Males have a single X and a tiny Y.

The X bears about 1, genes with varied functions. But the Y has hardly any genes; maybe 50, and only 27 of these are in the male-specific part of the Y. Many are present in multiple copies, most of them inactive, lying in giant loops of DNA. Thus the human Y shows all the signs of a degraded chromosome near the end of its life. We know that at 12 weeks an XY human embryo develops testes, which make male hormones and cause a baby to develop as a male.

Some reptiles, fish and frogs are XX female: XY male like humans, but have different sex genes. Other vertebrates, such as birds and snakes, are just the opposite, with ZZ males and ZW females, and the sex gene is different again. Many reptiles and some fish use environmental cues usually temperature rather than genetic triggers to determine sex. But back in the world of humans: Our sex chromosomes were once just a pair of ordinary chromosomes, which they still are in birds and reptiles.

We found they are still ordinary chromosomes even in monotreme mammals platypuses and echidnas which last shared a common ancestor with humans million years ago. This means that within the past million years the human Y lost most of its 1,odd genes, a rate of nearly 10 per million years. At this rate, the Y chromosome will disappear in about 4. This back-of-the-envelope calculation, inserted as a throwaway line in a little paper in , produced a hysterical reaction and loads of responses. When I talk about the disappearing Y, men in the audience shrink into their seats to protect their manhood.

But why the surprise? Degradation is typical of all sex chromosome systems. Acquisition of a gene that determines sex is the kiss of death for a chromosome, because other genes nearby on the Y evolve a male-specific function, and these genes are kept together by suppressing exchange with the X. The poor Y chromosome is also at a disadvantage because it is in the testis every generation. This is a dangerous place to be because cells must divide many times to make sperm, so mutations are much more frequent.

Of course, the loss of genes from the Y is unlikely to be linear. It could get faster as the Y becomes more unstable, or it could stabilize as the Y is stripped to [essential genes] http: In fact, humans have lost very few genes in the 25 million years since we diverged from monkeys.

So has the human Y finally stabilised? Maybe loss of any of the remaining 27 Y genes would compromise the viability, or fertility of the bearer. But looking more widely reveals that even genes on the human Y with important functions such as making sperm are missing from the mouse Y, and vice versa. Most spectacularly, species in two rodent groups have lost their entire Y chromosome. So it must be possible to dispense with the Y and start over again. If the human Y disappears, will men disappear?

So does that mean humans will become extinct in 4. The Y-less rodents have evolved a new sex determining gene, so why not humans? Perhaps this has already happened in some small isolated population, where genetic accidents are much more likely to take hold. Children of, say, an XX woman and a man with a novel sex gene, are likely to be intersex or at least infertile.

Such a reproductive barrier can drive incipient species apart, as happened with Y-less rodents. So if we return to Earth in 4. In any case, 4. We have been human for less than , years. And I can think of several ways in which we are likely to become extinct long before we run out of Y chromosome.

Available editions United Kingdom. Jenny Graves , La Trobe University. Jenny Graves , Author provided The X bears about 1, genes with varied functions.

So we are wrong if we think sex determination in human babies is typical of vertebrates. The degrading human Y But back in the world of humans: Has the human Y stabilised? A world without men? Three species of whiptail: Human evolution Evolutionary biology Human genetics Y chromosome.

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Other vertebrates, such as birds and snakes, are just the opposite, with ZZ males and ZW females, and the sex gene is different again. Many reptiles and some fish use environmental cues usually temperature rather than genetic triggers to determine sex.

But back in the world of humans: Our sex chromosomes were once just a pair of ordinary chromosomes, which they still are in birds and reptiles. We found they are still ordinary chromosomes even in monotreme mammals platypuses and echidnas which last shared a common ancestor with humans million years ago. This means that within the past million years the human Y lost most of its 1,odd genes, a rate of nearly 10 per million years.

At this rate, the Y chromosome will disappear in about 4. This back-of-the-envelope calculation, inserted as a throwaway line in a little paper in , produced a hysterical reaction and loads of responses.

When I talk about the disappearing Y, men in the audience shrink into their seats to protect their manhood. But why the surprise? Degradation is typical of all sex chromosome systems. Acquisition of a gene that determines sex is the kiss of death for a chromosome, because other genes nearby on the Y evolve a male-specific function, and these genes are kept together by suppressing exchange with the X.

The poor Y chromosome is also at a disadvantage because it is in the testis every generation. This is a dangerous place to be because cells must divide many times to make sperm, so mutations are much more frequent.

Of course, the loss of genes from the Y is unlikely to be linear. It could get faster as the Y becomes more unstable, or it could stabilize as the Y is stripped to [essential genes] http: In fact, humans have lost very few genes in the 25 million years since we diverged from monkeys. So has the human Y finally stabilised?

Maybe loss of any of the remaining 27 Y genes would compromise the viability, or fertility of the bearer. On one end of the short Y chromosome, there is a region thought to contain a gene called TDF testes determining factor that starts the process of an embryo becoming a boy and also releases an anti-Mullerian hormone AMH that suppresses the development of feminine structures.

This region is called the SRY sex determining region on the Y chromosome. All humans start off with the same basic body plan the evolutionist Stephen J. Gould, writing in Bully for Brontosaurus, concludes that this conserves energy within the species and also explains why males have nipples. The genital bud can become either a clitoris or a penis and the gonadal ridge can become either ovary or testes.

If the SRY is present, then the process of development leads to a male. If the SRY is absent, then the embryo will become female. If during meiosis the SRY ends up crossing over to the X chromosome, then an XX offspring normally a female will end up developing into a male because it has the SRY that causes maleness. How do scientists look at chromosomes? Visit gallery Play Thingdom Find out more.

How do you become you? Where did your genes come from? Who do you share genes with? The end of men? Which genes influence your sex?

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