Extinction of the human male

http://creation.com/y-chromosome-extinction

In 2003, Oxford University geneticist Bryan Sykes claimed that the human Y chromosome was “crumbling before our very eyes”. He warned that the demise of men was imminent.² However, doomsday predictions of Y-chromosome decay may have been a little hasty.

Compared to most chromosomes, Y is rather small. It has about 70 million base pairs and houses about 78 protein-coding genes (of the estimated 25,000 in the human genome).

Why is the Y chromosome in such peril?

Since the Y chromosome doesn’t have a ‘partner’, it cannot engage in a process known as genetic recombination. During meiosis, chromosome pairs line up, join and swap genetic material in a process called ‘recombination’. This is why two parents can have lots of children that are physically different from each other; the individual sets of chromosomes that each parent passes on are unique, highly-shuffled versions of their own chromosome pairs. This shuffling process enables a mutated chromosome to purge itself from some harmful mutations (see figure 1). But since the Y chromosome does not undergo recombination, mutated portions of it cannot be cut-and-paste over with a ‘healthier’ version. Thus Y chromosome mutations supposedly keep piling up. This inability of the Y chromosome to engage in recombination is one of the key reasons fuelling belief about its demise.

Moreover, the Y chromosome is supposedly bombarded by more mutations. Men produce sperm throughout their life, whereas women have a set number of egg cells at birth. This means that when men reproduce, their sperm has gone through more rounds of cell divisions, which means there’s more opportunity to accumulate mutations.

urthermore, evolutionary assumptions have boosted claims about the demise of the Y chromosome. The X and Y chromosomes are believed to have been a standard pair of non-sex chromosomes (autosomes) 300 million years ago. Since this time the X chromosome has supposedly maintained most of its genes, whereas the Y chromosome has decayed and shortened dramatically. That’s why this chromosome is often referred to as a ‘profoundly degenerate X chromosome’.

Y-demise proponents have also pointed out that many of the genes it contains have been rendered non-functional by mutations. Bryan Sykes calls it a “graveyard of rotting genes.

Self-healing chromosome

Although there may appear to be little future left for the Y chromosome, further research has revealed previously unsuspected ways of self-repair. The Y chromosome’s ability to heal itself is due to its long palindromic sequences (sequences that read the same in either direction). The Y chromosome contains eight large palindromes with genes imbedded in them—the largest is almost 3 million DNA ‘letters’ from end to end. These have earned the Y chromosome the nickname ‘a genetic hall of mirrors’.

So how do these help the chromosome repair itself? If a gene in one arm of a palindrome is corrupted by mutation, the middle of the palindrome can act like a hinge, bringing the two arms together. Then, in a process known as gene conversion, the ‘healthy’ gene in the complementary arm overwrites and restores the sequence in the mutated gene (see figure 2). This process helps explain why intact genes tend to reside in the palindrome arms, whereas the corrupted copies of these genes reside elsewhere.

How the Y chromosome heals itself. A – One of the palindromes has a mutated gene (M) and a normal copy of the gene (N) in opposite ‘arms’. B – The middle of the palindrome acts like a hinge, bringing the two genes in close contact. Gene conversion restores the mutated gene. C – Both arms of the palindrome end up with normal copies of the gene.

Conclusion

What the Y chromosome is telling us is that the neo-Darwinian mechanism of mutation and selection consistently degrades genetic software, as opposed to upgrading it. Though males are not doomed in the way Sykes claims, overall genome decay is a real phenomenon, and the more we appreciate the extent of the problem, the more it undermines the validity of the big picture of evolution.