Why the chromosome "male" Is it neglected by geneticists and… threatened?

As we have known since 2001, discoveries in genetics sometimes tend to be slightly… oversold. At the time, the teams of the Human Genome Project had announced to the whole world the complete sequencing of the human genome. In reality, it was a first draft, certainly of exceptional importance, but still full of holes. Since then, the white pages have gradually been filled in, and this spring, a giant step has been taken. Thanks to a new technology, a group of scientists was able to establish a first almost complete mapping of our genetic heritage. With one exception, and not the least, the Y chromosome. the Imagine Institute (Institute of Genetic Diseases).

This chromosome, which determines the male sex, therefore always presents unknowns. Nothing sexist here, though. Rather a technical facility on the part of geneticists: to simplify their work, they used a sample recovered after an early miscarriage, where an egg without a nucleus was fertilized by a sperm. Each chromosome is then present in only one copy instead of the usual two. For the sex chromosomes, it turns out that it was the X that was in this sample, not the Y.

Besides the satisfaction of having – finally! – a really complete reading of our genome, what could be the use of deciphering this last piece of our DNA? The answer should interest, and probably also worry a little, male readers: “We could perhaps understand why this chromosome, which is already the smallest of all, is dangerously reduced in size”, summarizes Professor Lyonnet. Indeed, specialists believe that within a few million years, it could even disappear completely. And after all, why not, since it appeared in reptiles only 300 million years ago (until then, their sex depended on their incubation temperature).

If the mystery remains on the reasons for this evolution, it is also because this chromosome, despite its small size, is also very difficult to read. There is a high concentration of what geneticists call “repeated sequences”, specific sequences of letters that previous generations of sequencers were unable to order correctly and therefore to decipher. “This is probably where we will understand why the Y chromosome turns out to be more unstable, more fragile than others”, hopes Stanislas Lyonnet.

Beyond this fundamental knowledge, this new information on “Y” would also be of medical interest, for anomalies and malformations linked to disorders of sexual differentiation. “In particular the whole question of intersex, when it is not always possible to classify the phenotype of sexual characteristics, and when it is difficult to assign a gender to a child”, explains the researcher. We bet that this last chapter of the book of life will be published quickly.

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