For many years the theory that the Y chromosome is degrading has been popular amongst geneticists. The theory states that the genes on the Y chromosome, found only in men, is degrading at an exponential rate and will lead to extinction in 5,000,000 years.
However, new research from the US indicates the the decay or degradation of the Y chromosome has completely ended.
Human sex determination is caused by either the presence or absence of the Y chromosome. The Y chromosome itself contains an SRY region (sex-determining region Y), which contains the genes for testis development factor (TDF). The protein coded by that gene causes the formation of testicles and has a role in the presence of other male characteristics and has a direct inhibiting effect on female sex characteristics.
As a result, when someone is born without a Y chromosome, they are genotypically (based on their genes) a woman. However, there is a condition known as “XX male syndrome,” which creates genotypically female women who are phenotypically male – they are still technically female though.
The Y chromosome is typically smaller than the X and contains only 78 genes vs. 800 in the X chromosome. It was also thought that the Y chromosome was genetically empty for many years.
The old belief is based on a statement made by Prof. Jennifer Graves of the Australian National University – in short it stated that the rate of decay of the Y chromosome indicates that it will cause extinction of the male sex in under 5,000,000 years. Prof Brian Sykes estimated it would be less than 100,000 years in his book – Adam’s Curse: A Future Without Men.
The reason for the initial belief and the reason why women will not go extinct is to do with the nature of sex determination. An embryo usually has only two sex chromosomes – an X and an X (female) or an X and a Y (male). When there is an XX combination (female), the two genes can undergo recombination and thus will not decay. Generally one X chromosome will deactivate and is visible with a light microscope in the nucleus of all somatic cells in a female as a Barr Body.
The Y chromosome cannot recombine with the X in males and thus is left open to decay. Therefore, any deletions that occur during the replication process will be forcibly passed on from father to son.
However, a study in the journal, Nature, from 2005, compared the lineage of the Y chromosome of humans with chimpanzees. Our two species diverged roughly 6,000,000 years ago and based on their degradation we can determine whether ours is faster or slower. Recently, the Rhesus Monkey’s Y chromosome was sequenced (diverged from us 25 million years ago). The conclusion was that we have not lost any genes in the last 6 million years and lost only one in the 19 million years prior to that!
Whoever needed a better reason to celebrate?