Razib Khan One-stop-shopping for all of my content

March 29, 2018

Ancestry does not always match up with appearance

Filed under: Human Genetic Variation,megan bowen — Razib Khan @ 9:04 pm

A few years ago I watched a bunch of Megan Bowen’s YouTubes about living in Korea as an expat. In one episode she had explained that the reason she had a black American accent (she’s from Georgia I think) is that she is a black American. Just a very light-skinned one.

In other videos, you can see that her skin is a little darker without typical Korean makeup, though she is still very light-skinned. And her natural hair is quite curly. But it would not be implausible to assume that she is one of the 10% or so of African Americans who are more than 50% white.

I didn’t think much about this until today. As part of my job, I watch ancestry-related YouTube videos to get a sense of how people interpret their results, and Megan Bowen showed up!

So I watched her video. There are some photos of her parents, and both look darker in complexion and more typically African American in their appearance. She also admitted that she was so light at birth that her father took a paternity test, and she was his.

The results for her ancestry came back…and she’s 65% Sub-Saharan African! This is curious because arguably Megan Bowen looks more “white” than the actress Megalyn Echikunwoke, who is 50% European (American) and 50% Nigerian (or half-Shona half-English Thandie Newton, the list could go on).

We have the genome-wide data. Megan is 65% Sub-Saharan African. And ~32% European.

Ultimately this is a pretty clear issue of the fact that only a subset of genes are responsible for the features which we deem ancestrally informative in a naive manner. Skin color, hair form, and facial features.

To the right is a plot from a paper which looked for variants affecting skin color in a Cape Vedre sample. They used ~900,000 SNPs to assess ancestry, so you know that that’s right. They also used a melanin index generated with a spectrophotometer. You see that 44% of the variation in skin color can be predicted by ancestry in this admixed population.

There’s a clear correlation between ancestry and complexion, but because the number of loci affecting the variation of complexion in humans is relatively small for a polygenic trait, the relationship can get decoupled rather easily (a few large effect genetic loci explain a lot of the rest of the variation).

If you looked at pigmentation loci in Megan Bowen and did local ancestry analysis, you’d see a strong enrichment for European segments. Far greater than the genome-wide 32%. It happens. It’s probability, not magic.

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