As an added bonus, the loss of the Y chromosome could lead to the evolution of new human species.

Y chromosome alone

The human blueprint lies in our cells. There are 20,344 genes on 23 pairs of chromosomes. Each pair contains one chromosome that we inherit from our mothers and another that we inherit from our father.

Two chromosomes in a pair are almost identical and carry the same genes. Therefore each cell contains two copies of each gene. If one copy is defective, the other can still keep the cell functioning.

But among the 23 pairs of chromosomes – the sex chromosomes – one stands out. The sex chromosomes are the same in women: two X, but in men the pair consists of an X chromosome and a Y chromosome.

This is a problem when a man produces sperm.

In the testes, “meiosis” occurs, a process in which all chromosomes join their partners, exchange their genes and eliminate mutations. But the Y chromosome has no partner to exchange anything with. Therefore, it shrinks as negative mutations accumulate.

A little simpler: a woman ensures that the female sex chromosome remains healthy with the pair of X chromosomes, while a man does not do the same for the Y chromosome.

The Y chromosome has become a shadow of itself. In our early mammalian ancestors, it contained about 1,500 genes, but now only 50 remain active.

One of them always kept the man alive. The so-called SRY gene, located on the Y chromosome, ensures the development of the testicles in the male fetus by producing proteins that activate the SOX9 gene (located on chromosome No. 17).

Without the Y chromosome, all embryos grow into females. This will be the end of the species unless the function of the Y chromosome is replaced. This is exactly what has happened in five species of mammals.

A new path to the testicles

One of the five species is the rodent Tokudaea osimensis, which lives only on Amami Oshima Island, 380 kilometers south of Japan’s main island.

The mouse lacks the Y chromosome and the SRY gene, but still produces males. The mystery is how he was able to stimulate the activity of the SOX9 gene and thus the development of the testicles.

To find an answer, a research team led by biologist Asato Kuroiwa compared the genomes of three females and three males.

like him pale Males have an extra copy of the DNA sequence on chromosome 3, where the SOX9 gene is located in rodents. The sequence, called Enh14, is supposed to promote SOX9 activity and thus testicle development.

To prove that the extra copy of Enh14 was indeed the cause of the development of male characteristics, the researchers inserted the DNA sequence into the embryos of test mice that had XX sex chromosomes – i.e. females – which immediately began developing testicles.

The conclusion was clear: copies of Enh14 stimulate the activity of the SOX-9 gene to such an extent that it replaces the Y chromosome and the SRY gene.

For Kuriwa, the result came after nearly 20 years of fighting to get permission to conduct research on endangered species.

“My ultimate goal was to find a sex determination mechanism that did not depend on the Y chromosome,” she says. Science in pictures.

Kuroiwa wants to investigate whether other DNA sequences play a role in sex determination in rodents. She also hopes to access tissue samples from four other rodent species that do not have a Y chromosome.

One is a closely related species on the neighboring island, which probably uses the same mechanism.

The other three species are closely related to the Japanese rat, and live in Eastern Europe and Central Asia. They may have other ways of surviving without the Y chromosome.

Y deficiency leads to the emergence of new species

Alternative mechanisms for male formation arose with the decline of the Y chromosome. Species populations split, causing some animals to use classical Y chromosomes and others to use new methods.

This division isolated the groups from each other genetically, because they were unable to produce offspring together, which automatically led to the evolution of different species.

If humans also lose the Y chromosome, we can expect a similar evolution, believes Australian geneticist Jenny Greaves.

She predicts that the Y chromosome will disappear completely within 11 million years. By then, people can look very different.

“If someone visited Earth in 11 million years, they wouldn’t find humans here, or multiple human species separated by different sex determination systems,” Greaves says.

The loss of the Y chromosome would not mean the immediate end of humanity, but rather the beginning of an entirely new chapter.