As there is a potential for them to be more readily purged in hemizygous males (and in cell lineages in females with the deleterious-allele-bearing chromosome activated), I would expect the frequency of deleterious alleles to be lower on this chromosome than in autosomes. Is this information known?

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    $\begingroup$ Just to make sure I understand the question. When you say "Is this information known", you actually ask "Are empirical observations coherent with this hypothesis?" $\endgroup$
    – Remi.b
    Mar 5 '14 at 10:08
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    $\begingroup$ Yes, that's at least in part what I mean. Thank you, @Remi.b. I imagine other hypotheses might also explain this empirical observation, if it indeed exists. I would also be interested to know whether this particular hypothesis has found stronger support than alternative hypotheses. But I would be most curious, as you say, about whether empirical data supports this prediction. $\endgroup$
    – Atticus29
    Mar 9 '14 at 3:09

"Monosomy for the X chromosome in humans creates a genetic Achilles' heel for nature to deal with."

The article I quoted1 tackles this issue. It's from the turn of the century, though, so quite old in sequencing/genetic terms, but the science is still sound as far as I can tell. Here's the money quote:

Hence, the X chromosome appears to have a lower gene density and should yield fewer deleterious mutations per megabase of DNA.

In short, yes, you're correct. There appears to be less overall information on the X-chromosome, deleterious or not, and selection is faster; the X chromosome is just flat-out less diverse. This article goes into quite a bit more but says essentially the same thing: "On one hand, the greater effectiveness of selection on males due to the hemizygous expression of the X, which may be further reinforced by sexual selection, is expected to lower the mutation load for females at shared loci." Here is a more recent article stating the same: "This finding [X-chromosomes with lower linkage disequalibrium] is consistent with more rapid selection against deleterious mutations when the X is monosomic in males, and to a lesser extent, under random inactivation (Lyonization) in females."

Finally, here is a piece written a few years back by a colleague of mine that goes into depth on the subject. The second half deals with population differences (also fascinating), but the first part has a lot of good detail on what you want.


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