Tuesday, February 24, 2015

Malodorous, hmm ...

From the Wikipedia article: today we are talking about the gene ABCC11 and its alleles, which have the following unfortunate consequences.
"The product of this gene participates in physiological processes involving bile acids, conjugated steroids, and cyclic nucleotides. In addition, an SNP in this gene is responsible for determination of human earwax type and presence of underarm odour."
My earwax type, like most Europeans, is 'wet earwax'. To be honest I could be happy with no earwax at all - lose the regular eardrops and self-syringing! Here are the alleles of the ABCC11 gene as segregating at SNP locus rs17822931 according to SNPedia.
Wet earwax. Normal body odour. 
Wet earwax. Slightly better body odour.
Dry earwax. No body odour. Likely Asian ancestry. 
They're very polite over at SNPedia. Here's what Razib Khan noted in this post.
"The subjects were a few hundred Filipins. This is a population where the allele of interest segregates in intermediate frequencies. So there are many individuals with dry earwax as well as wet earwax, and all the associated traits.

Here are some tables I extracted*:

Mean malodour scores
5 hours24 hours
Uses deodorant0.50.860.97
Does not use0.50.140.03

"I have no idea how subjective malodour scales work, but the moral is pretty straightforward. Those with the TT genotype saturate at a much lower point. This manifests in daily behavior. There is a fair amount of Japanese data that people who go to the doctor for body odor issues are much more likely to have wet earwax. This data from the Philippines illustrates that individuals with the derived genotype, TT, must be conscious enough of their lack of body odor to forgo deodorant purchases, even though I assume it is normative in the American influenced culture of the Philippines."
My own 23andMe data returns this:

My 23andMe data for the ABCC11 gene

Keep those showers coming and don't forget the Sure is, I think, the message here.


The real puzzle is how the relatively recent (c. 50,000 years ago) T mutation swept to near fixation in the Han Chinese and neighbouring populations. What on earth was the selective advantage? *

From the comment by Greg Cochran at Razib's post:

"It’s essentially impossible for selection to favor a pure recessive trait. You’re never going to see it. Therefore, we can conclude that some effect of this mutation shows up in heterozygotes and is favored by natural selection – but nobody seems to know what it is."

Why is it "essentially impossible for selection to favor a pure recessive trait"? The recessive mutation is initially rare in the population and the offspring of the carrier will almost certainly be heterozygous. This hides the selective advantage of the recessive allele so it doesn't get to spread in the population. Most likely it will simply die out.


* Update: it could be sexual selection for smelling better: CT beats CC leading to heterozygote advantage, then TT wins out.