Scientists warn that men are going to become an extinct species. Y chromosomes are shortening and may disappear in several million years. What is the minimum size of the Y chromosome, with which it can preserve its functions?

The nucleus of a living cell contains several pairs of chromosomes. In sexual reproduction, one of the chromosomes in each pair is from the mother, and the other is from the father. There are several different mechanisms of determining the biological sex of a creature. One of them is based on a pair of chromosomes. For instance, human cells contain 23 pairs of chromosomes, one of which determines the sex.

All mammals, including humans, use similar mechanisms of sex determination based on X and Y chromosomes. A pair of two X chromosomes determines female sex, a combination of X and Y chromosomes determines male sex. Other species use other mechanisms; for instance, birds use Z and W chromosomes for sex determination, and in many reptiles sex depends on the incubation temperature rather than on combinations of chromosomes.

While the X chromosome contains around 2000 genes, the Y chromosome is one of the shortest and contains less than 50 genes, of which only 13 are functional. One of these genes, the SRY gene, triggers development of male sex in a fetus.

Several studies show that remote ancestors of modern humans had over 1400 genes in their Y chromosomes, but over millions of years of evolution it shrank to its current small size. The rate of shrinking was estimated as 4.6 genes per million years. This means that the Y chromosomes may lose all their functional genes or even completely disappear in several million years. And, with their disappearance men will disappear as well. Either this will be a return to asexual reproduction or another mechanism of sex determination will develop.

A group of scientists at the University of Hawaii decided to find out what is the minimum set of genes in the Y chromosome needed to produce male organisms. They conducted a series of experiments on mice, using in vitro fertilization and genetic engineering. In these experiments they established that only two genes are necessary for this, one of which is the SRY gene. With these extra-short Y chromosomes male mice are healthy (as far as scientists can measure their health – mice don’t tell scientists how they feel); the length of their life is the same as of other male mice. The only difference is that they cannot reproduce. Therefore, while an individual male can live with this short chromosome, the species as a whole cannot survive.

Other researchers believe that the estimation of the rate of shrinking of the Y chromosome is not correct. They insist that the shrinking was much faster 300 to 100 million years ago, and it has decelerated or completely stopped by now. Therefore, linear extrapolation is not applicable in this case, and it is too early to worry about disappearance of the Y chromosome.

This text is based on the information presented in the "Science in Action" BBC radio programme.
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