Sex determination at birth is decided on the chromosomes the baby has. A female has two X chromosomes, whereas a male has one X and one Y chromosome.
According to Medline Plus, in every cell, humans possess 23 pairs of chromosomes, among which the sex chromosomes constitute one pair. The X chromosome, comprising roughly 155 million DNA base pairs, makes up around 5% of the total DNA in cells.
The Y chromosome extends over more than 59 million DNA building blocks (base pairs) and accounts for nearly 2% of the total DNA found in cells.
However, according to The Conversation, the Y chromosome is degrading at a concerning rate.
The Conversation notes that there’s been plenty of changes to the Y chromosome over the years.
Our sex chromosomes were originally a pair of typical XY chromosomes, a trait still observed in birds and reptiles. Even in monotreme mammals, like platypuses and echidnas, XY chromosomes are ordinary.
Within the last 166 million years, the human Y chromosome has lost most of its 1,600 genes at a rate of nearly 10 per million years. At this pace, the Y chromosome is expected to vanish in about 4.5 million years.
This isn’t unexpected, according to The Conversation. It’s common for sex chromosomes to break down over time. “Acquisition of a gene that determines sex is the kiss of death for a chromosome, because other genes nearby on the Y evolve a male-specific function, and these genes are kept together by suppressing exchange with the X.”
This means the Y chromosome can’t swap out bad DNA with the X chromosome, making it hard to eliminate mutations, deletions or bad DNA.
“The poor Y chromosome is also at a disadvantage because it is in the testis every generation,” The Conversation continues. “This is a dangerous place to be because cells must divide many times to make sperm, so mutations are much more frequent.”
Some species of mole voles in eastern Europe and spiny rats in Japan have witnessed the complete disappearance of the Y chromosome and SRY gene, which helps determine the male sex in certain populations of animals. In these species, the X chromosome remains, existing in either a single or double dose in both males and females, per Science Alert.
In a study published by PNAS, Hokkaido University biologist Asato Kuroiwa and her team uncovered that most genes originally on the Y chromosome of spiny rats have relocated to other chromosomes. However, they had difficulty finding the SRY gene.
In 2022, they successfully identified DNA sequences present in male rats and not females. After studying the sequences closely, they also discovered a small difference near the gene called SOX9. This gene tells the body how to make a protein important for developing testes, per the study.
The scientists believe that this extra piece of DNA is like a switch that turns on SOX9, even without the SRY gene. To test their idea, they put this extra DNA into mice and found out that it made SOX9 work better.
This means that even without SRY, SOX9 can still do its job, thanks to this little piece of extra DNA, according to the study.
