The remarkable endurance of the Y chromosome, ‘master of maleness’

General Studies Paper 3

Context

• The Y chromosome, often referred to as the “master of maleness”, has long captivated scientists and historians alike.

Y chromosome

• In humans, in addition to the 22 pairs of chromosomes in each, we have a pair of sex chromosomes called X and Y.
• Sex as a specification is determined by these sex chromosomes. They carry sex-determining genes.
• All biological males have X and Y chromosomes and all biological females have two X chromosomes.
• The ‘sex-determining region Y’ on the Y chromosome determines the biological male sex.

‘Juvenile delinquent’

• Estimated to have emerged around 200-300 million years ago in a common ancestor of all mammals, the Y chromosome has had a unique genetic journey, and embedded within its DNA lies a remarkable tale of evolution.
• Scientists published the complete genetic sequence of the Y chromosome in 2003. This sequence provided an outline of 23 million bases of the 60 million or so bases that together make up the Y chromosome.
• In total, the chromosome encoded for only 55 genes and accounted for around 2% of the genetic material inside a cell.
• Many researchers jokingly refer to the Y chromosome as the “juvenile delinquent” among chromosomes pertaining to its abundance of repetitive sequences, poor functional utility (with a small number of genes), reluctance to socialise (i.e. recombine with other chromosomes), and a high proclivity to degenerate over the course of evolution.
• Indeed, because it has little potential to recombine, the diminutive Y chromosome has been passed from father to son, carrying the legacy of generations.

Vital genes

• In a landmark genetic study, published in March 2003 in the American Journal of Human Genetics, researchers reported that around 0.5% of all the men in the world have inherited a Y chromosome from the Mongol emperor Genghis Khan or one of his descendants.
• Y chromosome possesses genes that are vital to biological functions, including those linked to ageing and lifespan regulation.
• In the animal kingdom (including mammals), scientists have noticed substantial differences in lifespan between the sexes: the females tend to live longer than the males.
• This phenomenon has been attributed largely to the absence of a second Y chromosome in males, exposing the deleterious mutations in the X chromosome.
• It is also well known that men lose the Y chromosome with age and that this is associated with a higher frequency of cancers, Alzheimer’s disease, and a shorter lifespan.

Losing the Y

• Studies have shown that LoY in humans occurs with age and is associated with several debilitating medical conditions – a finding that has been validated in mice with LoY, resulting in weak heart muscles (cardiomyopathy), stretchedor thickened heart tissue (fibrosis), and heart failure.
• researchers have also found that the pathological effects observed on account of LoY in mice’s hearts could be negated by transforming growth factor beta 1-neutralising antibodies, suggesting a potential treatment for this medical condition in future.
• The human Y chromosome is about one-third as big as the X chromosome. So, many animal species, including humans, have a genuine fear of losing the Y chromosome in the distant future.

Conclusion

• Genome sequences of the Neanderthals, an ancient relative of the modern human, harbour telltale signs of the replacement of the Y chromosome beginning from modern humans. This suggests that such replacement is not new to the human lineage, and that it is quite possible that the Y chromosome may have to relinquish its coveted title of “master of maleness” to another chromosome in the times to come.