Context: According to a new research by scientists at the University of New South Wales (UNSW) in Sydney, sex chromosomes is the key reason behind men living shorter lives than women worldwide.

About:

• 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.
• Sex chromosomes: 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 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. 

Result of hypothesis testing 

• 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.
• “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).