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, and scientists have proposed various theories as to why that is so — men take bigger risks, they drink and smoke more. Now, new research has tested one of many hypothesis — that the real reason is related to the sex chromosomes — and it appears to hold up.
The research, by scientists at the University of New South Wales (UNSW) in Sydney, was published on Wednesday in the journal Biology Letters.
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 maximises the length of life, according to the hypothesis. And this is what the UNSW researchers set out to examine.
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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. “We looked at lifespan data in not just primates, other mammals and birds, but also reptiles, fish, amphibians, arachnids, cockroaches, grasshoppers, beetles, butterflies and moths among others,” she said. “And we 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), and it’s 17.6 per cent earlier on average.”
While the pattern is the same across species, the sexes affected are sometimes the reverse. In birds, butterflies and moths. it is the male that has an identical pair of sex chromosomes (ZZ) while the female has ZW chromosomes. Female birds, butterflies and moths were usually found to die earlier than their male counterparts, giving more credence to the unguarded X hypothesis (strictly speaking, unguarded Z in this case).
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“In species where males are heterogametic (XY), females live almost 21% longer than males. But in the species of birds, butterflies and moths, where females are heterogametic (ZW), males only outlive females by 7%,” Xirocostas said.
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