A woman has two X chromosomes, while a man has one X and one Y chromosome. According to Medline Plus, in each cell, humans have 23 pairs of chromosomes, of which heterosomes are one pair. The X chromosome, which has around 155 million DNA base pairs, represents about 5% of the total DNA in cells, Deseret News reports.
The Y chromosome could disappear in about 4.5 million years. Photo: Archive
The Y chromosome spans more than 59 million DNA pairs (base pairs) and makes up nearly 2% of all DNA in cells.
However, according to The Conversation, the Y chromosome is in a worrying process of degradation. The same source states that over the years, the Y chromosome has undergone a lot of changes.
The sex chromosomes were originally a pair of typical XY chromosomes, a feature still seen in birds and reptiles. Even in monotremes such as platypuses and echidnas, XY chromosomes are common.
Over the past 166 million years, the human Y chromosome has lost most of its 1,600 genes, at a rate of nearly 10 every million years. At this rate, the Y chromosome is expected to disappear in about 4.5 million years.
This is not at all surprising, according to the cited source. It is common for sex chromosomes to break down over time. “The acquisition of a sex-determining gene is the “kiss of death” for a chromosome, because other genes near the Y evolve with a male-specific function, and these genes are held together by eliminating the exchange with the X.”
This means that the Y chromosome cannot exchange inappropriate DNA with the X chromosome, making it difficult to remove mutations or inappropriate DNA.
“The poor Y chromosome is also at a disadvantage because it is in the testicles every generation“, The Conversation continues. “This is a dangerous place because cells have to divide many times to produce sperm, so mutations are much more common“.
What happened to the Y chromosomes of animals
Certain species of field mice in Eastern Europe and spiny rats in Japan have witnessed the complete disappearance of the Y chromosome and the SRY gene, which helps determine male sex in certain animal species. In these species, the X chromosome remains, being present in single or double doses in both males and females, according to Science Alert.
In a study published by PNAS, biologist Asato Kuroiwa of Hokkaido University and his team found that the vast majority of genes originally present on the spiny rat Y chromosome were found on other chromosomes. However, they had difficulty finding the SRY gene.
In 2022, they succeeded in identifying DNA sequences present in male rats but not in female rats. After carefully analyzing the sequences, they also discovered a small difference near a gene called SOX9. This gene tells the body how to make a protein important for testicular development, according to the study.
Scientists believe that this extra piece of DNA is a kind of switch that activates SOX9, even without the SRY gene. To test their idea, they introduced this extra DNA into mice and found that it made SOX9 work better.
This means that even without SRY, SOX9 can do its job, thanks to this little extra piece of DNA, according to the study.
Is there hope?
The Conversation notes that biologist David Page's team in Boston is a strong proponent of the durability of the Y chromosome.
They point out that although chimpanzees have lost some genes since humans and chimpanzees shared a common ancestor 5 million years ago, humans have not experienced significant gene loss. In fact, in the 25 million years since humans diverged from apes, humans have lost very few genes.
“There are genes on the Y chromosome that are active in every corner of the body. Skin, blood, brain, lungs – whatever. They seem to be some kind of global entrepreneurs within the human genome. They are kind of master regulators”Page told NPR.
Chromosome loss and disease in men
Jan Dumanski of Uppsala University in Sweden told NPR that there may be a link between the Y chromosome and cancer.
A review released by the National Library of Medicine found that the loss of the Y chromosome could also lead to an increased risk of developing Alzheimer's disease, cancer and cardiovascular disease.
This research does not include health data to show what would happen if genes on the Y chromosome were moved to another chromosome, as happened in the spiny rats in Kuroiwa's study.
