Is the y-chromosome in humans destined to disappear?

I think there are probably few matters that men tend to worry more about than their masculinity. When years ago I saw the announcement of a public talk on “The Disappearance of the y-Chromosome”, I simply needed to hear what that was all about. Since I listened to that talk at that time, a great deal more research data are available on that topic. Fact is that all “maleness” is conferred by the y-chromosome and “femaleness” by the X-chromosome, both being termed “sex chromosomes”. But while women have two X-chromosomes (= XX), men only possess one X and a much smaller y-chromosome about one fourth the size of the X (=yX). Chromosomes (of which all humans have 46  -not counting chromosome number anomalies) occur in identical pairs and that holds true also for the two X-chromosomes in a woman. However, a man’s single y-chromosome’s partner is an X and because of their much different sizes the two chromosomes cannot recombine and repair each other if some bad mutation has occurred on one of them. Think of heaemophilia, which is due to a bad gene on the X-chromosome, but doesn’t make a female ill if she has another “healthy” X. Men do not have that luxury and develop the sickness.

Now assume a bad mutation occurs on the y-chromosome and there is no chance to repair or compensate for it? The only way to weed out the bad gene is to lose it, to delete it and by having to do this repeatedly over millions of years, the y-chromosome is bound to get smaller and smaller. It now contains no more than perhaps 50 genes, while the X-chromosome contains about 20 times that many. According to some researchers, like world famous Australian geneticist Dr Jennifer Ann Graves, about 300 million years ago sex-chromosomes were not yet terribly different from the other chromosomes known as autosomes, but with one of them (the one carrying the gene for maleness) beginning to lose genes, the fateful decline in genes took its course and from about 200 million years ago saw the development of differently sized X and y-chromosomes plus a loss of about 10 genes every one million years. With only 45-50 genes left on present day human y-chromosomes, one can expect the y-chromosome to have disappeared in 4-5 million years. Not so, according to Dr Jenn Hughes, who argues that gene loss affecting the y-chromosome in humans is not constant as only one gene disappeared in the last 25 million years.

However, even she has to admit that ultimately there is the possibility that in many, many millions of years to come, human males (if humans are still around) could be without their little “y”, but not necessarily without the organ many men are usually so worried about. There are, after all, already a few species of mammals, for instance the famed but endangered spiny rats of Amami and Oshima islands in Japan, in which males have no y-chromosome at all. Since the offspring of these rats is not hermaphroditic, but differentiates into male and female individuals (in which the brain of male rats expresses genes to produce hormones that are involved in seminal vesicle protein 5 [Svs5] and cytochrome P450 1B1 [Cyp1b19] syntheses, but the brain of female rats upregulates serine or cysteine peptidase inhibitor and other molecules typical for females), some chromosome other than the “y” must have taken over its function. Yet, what, when, and how exactly that happened is still uncertain.

Virtually all of the few genes still present on the y-chromosome are “maleness”-genes and the SRY-gene is the most important one, as it causes the development of external and internal male genitals. Other genes are involved in regulating sperm and seminal fluid production. How important the y-chromosome is, one can see in cases of people with a multiple X and one single y-chromosome: even XXXy individuals have male genitalia! And one more important aspect of the y-chromosome: because of mutations on it, it allowed researchers to track people’s migration routes out of Africa from 100,000 years ago to now.

© Dr V.B. Meyer-Rochow and, 2021.
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