Question

Imagine we have two diploid species which diverged 10 million years ago. On sequencing part of an autosome where we think most of the mutations are neutral we find that they are 20% divergent.

Give 2 explanations as to why the rates of mutation differ on the autosomes and X-chromosome?

Answer 1

Reason for being difference in the mutation rate of autosomes and X-chromosomes is due to:

1. Genes on autosomes spend an equal amount of their time in males and females, so that their net mutation rate is average of male and female mutation rates. While X-linked genes spend only one-third of there time in males and two-third of their times in female. And the mutation rate is equal to the divergence rate. If the mutation rate is higher in case of spermatogenesis than oogenesis than autosome will show more divergence than X-chromosome or vice-versa.

2. Low mutation rate in case of X-chromosome is due to, their capability to evolve towards reducing the deleterious recessive mutation in hemizygous males (XY).