Monday, June 18, 2012

Accessible research: Male mutation bias

One of the topics I study is male mutation bias. In females all of our eggs are formed at birth (minus the last meiotic division, which happens monthly in humans), while males continue to produce sperm throughout their lifetime. If most mutations occur through errors during replication, then we expect more mutations to originate in the male germline (which is continually replicating to produce new sperm) than in the female germline.

One way we can study whether there are differences in the female mutation rate and the male mutation rate by looking at the accumulation of fixed mutations (called substitutions) on chromosomes that spend different amounts of time in the female and male germline. For example, the non-sex chromosomes (autosomes) are present in two copies in females, and two copies in males, and so are equally affected by the female mutation rate (Uf) and male mutation rate (Um). Alternatively the X chromosome is present in two copies in females, but only one copy in males, so represents 2/3 of the female mutation rate, and 1/3 of the male mutation rate. And, the Y chromosome, being present only in males, represents only the male mutation rate. 
By comparing substitution rates on any two pairs of chromosome types (X/A, Y/A or X/Y), and doing just a little math, we can solve for the male-to-female mutation rate ratio (Um/Uf = alpha). If alpha is equal to 1 then the male and female mutation rates are equal, and there is no bias. If alpha is greater than 1 then we can deduce that more mutations come from the male germline than the female germline.

So far alpha has been equal to or greater than one in all mammals studied. How cool is that?! There are many implications of male mutation bias, and I'll talk more about this in an upcoming post.

What inspired me, after so long, to finally give the background to one of my areas of interest? Was it interest in furthering your understanding of the science that I'm passionate about? Was it a general interest in promoting scientific literacy? Was it my enthusiasm for my research? Well, yes.

But most of all, I wanted to help you understand why this cartoon is especially funny to me.

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