A new paper on Chinese genomics using hundreds of thousands of low-coverage data from NIPT screenings is making some waves. I’ll probably talk about the paper at some point. But I want to highlight the frequency of rs17822931 in Han Chinese. It’s pretty incredible how high it is.
Because the derived variant SNP, which is correlated with dry flaky earwax when present in homozygote genotypes, is also associated with less body odor, it has been studied extensively by East Asian geneticists. Basically, individuals who are homozygote for the ancestral SNP, which is the norm in Europe, the Middle East, and Africa, tend to have more body odor, and in societies and contexts where this is offensive these people are subject to more ostracism in East Asia as they are a minority (some of the studies in Japan were motivated by conscripts who elicited complaints from their colleagues).
The relatively low frequency in Guangxi is to be expected. This province was Sinicized only recently. As in, the last 500 years. And it still retains a huge ethnic minority population, and many of the Han in the province likely have that ancestry. But the question still arises: why do the Han have such a high frequency of rs17822931?
In human populations a SNP in ABCC11 is correlated with two salient traits: 1) wet or dry earwax 2) body odor. When I had my first son sequenced before his birth the main variant of phenotypic consequence that I noticed (aside from him being a heterozygote on KITLG), was that he carried a derived mutation on this position. Meaning that he was going to have dry earwax and fewer issues with body odor.
My wife and I are both heterozygotes. This is not too surprising. The derived variant is actually greater than 50% in Bengalis in the 1000 Genomes (in South India the derived variant is also around ~50%), while about ~25% of Northern Europeans are heterozygotes.
There’s another reason few Chinese consumers buy deodorant: basic biology.
Scientists in recent years have shown that many East Asians, a group that includes China’s ethnic Han majority, have a gene that lowers the likelihood of a strong “human axillary odor” — scientist-speak for body stink.
That lowers the likelihood that they will use deodorant to begin with, according to a 2013 study by researchers at the University of Bristol and Brunel University in Britain, after a survey of nearly 6,500 women of various backgrounds.
“It is likely that deodorant usage is not widely adopted because there is, for much of the East Asia population, no need for it,” it said. (For those curious about such matters, that same genetic difference also leads to drier earwax.)
A friend of mine in undergrad of East Asian background told me once that she had never worn deodorant. So this shouldn’t be very surprising.
The correlation between carrying the G, ancestral, allele, and body odor is very strong. Though it is imperfect. Going through this literature human smells are clearly a polygenic trait (see The effect of ethnicity on human axillary odorant production). That being said, this case-control study in a Han population shows ABCC11‘s importance in at least East Asian populations (earlier work in Japan showed that those with body odor tended to have wet earwax and carry the G allele as well).
In regards to the genotype proportions the authors observe:
The excessive heterozygosity observed in AO individuals is probably due to the effect of selection, particularly nonrandom mating against AO phenotype.
This doesn’t make sense to me. Wouldn’t people who have body odor tend to pair up in a society where they are a minority? The authors note that the excess of heterozygotes was observed in earlier studies too.
If you dig into the frequencies it seems that the derived mutation is absent among populations in Africa without recent Eurasian back-migration. I looked it up, and it’s segregating in ancient Eurasian samples, with Ust Ishim being a heterozygote. It is curious that in no population has the derived frequency swept to fixation, nor has the ancestral variant fixed in other groups (such as in Europe).
I strongly doubt that there is any selection on this locus due to earwax or body odor. It is a pleiotropic locus, there are other effects from the mutation. One of those other effects is probably the target of any selection. And in regards to selection, it seems likely that that would be a balancing sort since neither the ancestral nor the derived variant are fixed in most populations.