More pigmentation genetics

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Pigementation is turning out to be one of the most tractable phenotypes for genome-wide association studies– a new paper from Decode identifies another couple loci that influence various aspects of hair and skin pigment. These add to the growing list of pigmentation genes in Europeans–I’ve not done the math, but these loci must account for some sizeable chunk of the total genetic variance in these traits.

Why has pigmentation been so amenable to mapping, while other traits like height, weight, or most diseases hover at about 2-3% of the variance explained? One possibility is that it’s because pigmentation has been subject to strong recent selection–theory predicts that the initial moves toward an new fitness optimum will be loci of large effects, while the later moves will be smaller. Perhaps since this selective pressure has been so recent, we’re still picking up those initial mutations of large effect that are still segregating in the population?

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8 Comments

  1. -I’ve not done the math, but these loci must account for some sizeable chunk of the total genetic variance in these traits.  
     
    there have been admixture studies with african americans for: 
    slc24a5 
    kiltg 
     
    the ‘melanin unit’ diff. between europeans and west africans is 30.  
     
    for slc24a5 ancestral homozyg. vs. derived homozyg. you get 9.5 units of difference. 
     
    for kitlg ancestral homozyg. vs. derived homozyg. you get 6 units of difference. 
     
    both these are somewhat ‘dominant’ in effects in terms of lightening. might be gene-gene interactions, but still, if you add these two you get around half of the difference between west africans and northern europeans. i wouldn’t be surprised if slc45a2/MAPT is nearly as big, but haven’t seen admixture study.

  2. Perhaps since this selective pressure has been so recent, we’re still picking up those initial mutations of large effect that are still segregating in the population? 
     
    I read somewhere that skin color is subject to cycles of fashion, so there was probably some 3000 BC equivalent of Coco Chanel who, just by chance (let’s say due to inclement weather that year), happened to stay inside most of the year. When she came out, the masses could not help but imitate her, selectively breeding their kids to be lighter in color, with assortative mating for skin color occurring in each subsequent generation, and skin color tied to social rank thereafter. 
     
    Of course, ever since the contemporary Coco Chanel caused dark skin to become fashionable, we have seen a reverse trend, and within 5000 more years, Europeans will have gone back to a dark complexion.

  3. I read somewhere that skin color is subject to cycles of fashion, so there was probably some 3000 BC equivalent of Coco Chanel who 
     
    but the sweeps started before then. slc24a5 might be young (4000 BCish), but i was told these estimates to under more than overestimate, and we know from tocharians that it is likely to be earlier than 6600 BP (you’ve read my posts). slc45a2 starting sweeping 11500 years ago. but the confidence intervals on these are big….

  4. OK, so the ancient Coco Chanel lived a few thousand years before that. Come to think of it, maybe it was about the time when people started depicting the human form in art — by chance, only lighter shades were available, and just as women imitate what they see in fashion magazines today, so it was then.

  5. assman, you are really backdating “globalization” here….

  6. (dude, it’s a joke)

  7. This important research will soon enable scientists to build a master race.

  8. Chaka, the Poles already exist.

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