Men outnumbered women by 37 million in the 2011 Census of India, but among those over the age of 60, there were more than 1 million more women than men. In general, men live shorter lives than women worldwide. This is due to the chromosomal differences between the two, points’ new study.

What are Chromosomes?

• The human body is made up of cells, and in the centre of each cell is the nucleus. Chromosomes, which are located inside the nucleus, are structures that hold the genes.
• It is the genes that determine the various traits of an individual including eye colour, blood type — and sex.
• The human cell has 23 pairs of chromosomes. One pair is of the sex chromosomes, named X and Y, which determine whether an individual is male or female.
• A female has two X chromosomes (XX) while a male has one X and one Y (XY).

Unguarded X hypothesis

• This hypothesis suggests that the Y chromosome in XY is less able to to protect an individual from harmful genes expressed on the X chromosome.
• In a male, as the Y chromosome is smaller than the X chromosome, it is unable to “hide” an X chromosome that carries harmful mutations, which may later expose the individual to health threats.
• On the other hand, the hypothesis goes, there is no such problem in a pair of X chromosomes (XX) in a female.
• If one of the X chromosomes has genes that have suffered mutations, then the other X chromosome, which is healthy, can stand in for the first, so that the harmful genes are not expressed.
• This maximizes the length of life, according to the hypothesis. And this is what the UNSW researchers set out to examine.

Testing the hypothesis

• In a statement issued by UNSW, PhD student and study first author Zoe Xirocostas said the
• Unguarded X hypothesis appears to stack up, after examining the lifespan data available on a wide range of animal species.
• Researchers studied lifespan data in not just primates but mammals and birds, but also reptiles, fish, amphibians, arachnids, cockroaches, grasshoppers, beetles, butterflies and moths among others.
• It was found that across that broad range of species, the heterogametic sex (XY in humans) does tend to die earlier than the homogametic sex (XX in humans).

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