Gene Expression

“Black” & white twins again

i-cc40adc19e04c674a4aa10874560a15b-twins.jpgHsien-Hsien Lei points me to another story about black and white twins. First, the “black” twin is clearly mixed race, her skin color is between the modal complexion of Europeans and sub-Saharan Africans on the von Luschan scale. The “white” twin on the other hand does seem to exhibit the color of someone of European descent. What’s going on here? This is somewhat different than the other case of black and white twins, in that case both parents were mixed-race, in this case the father is white (German) and the mother is mixed-race (Jamaican & English). The two cases are different even though the outcome is pretty much the same in appearence, which goes to show that there are many ways to skin the cat.

i-0687f10ab8b726830e549ac99c6f70ab-twinz.gifIn any case, you are getting the “million to one” odds story here. Let’s see how you get there. The math here is pretty simple. You know that there are 4-5 loci which control most of the skin color variation between geographic races. Very dark skinned peoples seem to exhibit a “consensus sequence” where all the genes are “on” and light skinned peoples often lose function so the genes are operationally “off.” To simplify the math, let’s assume there are 4 loci which result in the skin color variation between these twins and between Africans and Europeans. Humans are diploid, we have two copies, alleles, of each gene. In a simplified model dark-skinned sub-Saharan Africans would be fixed for functional alleles on all the loci, so you’d have 4 pairs “on.” Light-skinned Europeans would be “off” on all the loci, so you’d have 4 pairs “off.” Genetically, a mixed-race individual, an F1 hybrid, who is half-black and half-white would have alternating alleles on each locus because they would receive a functional allele from their African parent and a nonfunctional one from their European parent, so, you expect their skin color to be between their parental values if you assume additivity and independence of the average effect of the loci (this is close enough to reality). Now, if this mixed-race individual mates with someone who is European, what happens? In this case, the European father is going to automatically contribute a nonfunctional allele on every locus. In other words, for each locus the children are guaranteed one nonfunctional allele. This means that the variation will be all on the mixed-race mother, and the math here is very simple: you know she has an “on” and an “off” copy because one of her parents was African and the other European, so on each locus there is a 1 out of 2 chance for either allelic variant.

So, you have: (1/2 chance of “on” allele)4 loci = 6.25% chance that all loci will be “on”
So, you have: (1/2 chance of “off” allele)4 loci = 6.25% chance that all loci will be “off”

That means that in this pairing there is a 1 out of 16 chance that any given child will be as white as the father or as brown as the mother, with the expectation being in the middle. What are the chances of this particular outcome: one white twin, and one “black” twin?

Well, if each child is a fraternal twin the chances of the outcomes are independent, so you multiply across, 1/16 X 1/16 = 1/256, or one out of a 256 chances to get this combination, but…you have to double it because there is a case where the colors might be reversed, so it is actually 1 out of 128, or 0.8% of fraternal twins with this combination of parents would come out like this. So to get to 1 in a million you need to mutiply out by the fraternal twinning rate, and this exhibits interpopulational variance as well as dependency on diet. In Japan it is 1 in a 1000 while in some African nations it is 15 in 1000. Using the Japanese fraternal twinning rate you get 1 in 128,000 chance, and the African twinning rate a 1 in 8,500 chance. So not quite 1 in a million!

The only caveat I want to reiterate is that this case is dependent on cultural perceptions of race. The complexion of the white twin is clearly similar to that of Europeans, but the “black” twin is not in the color range of typical West Africans (the source population for Jamaica). It would simply be impossible for the offspring of a European father and a mixed-race mother to be as dark as a sub-Saharan African, but Western standards of “black” are considerably more elastic than “white.” In the previous story about black & white twins, the “black” twin again was not as dark as a sub-Saharan African, so it wasn’t actually as extreme a case of resegregation of alleles as the press made it out to be.

Update: Due to some comments I will be more specific about my assertion that the “black” twin is “clearly” mixed-race: the population of comparison I had in mind did not include all sub-Saharan Africans, but a subset derived from the coastal areas of West Africa. The reason being that the vast majority of the Africans in Jamaica were derived from this region. If the child on the left was born in Jamaica I suspect that based on phenotype the population would classify it as “mixed-race,” not black. This is not to say that some coastal West Africans do not exhibit the same phenotype (though in places like Accra there was also a non-trivial admixture with Europeans over the centuries) in regards to complexion, but I would hold that they would be a distinct minority, to the point where knowledge of the priors (e.g., that this child was born outside of Africa in the diaspora) would imply non-African ancestry. This is only relevant insofar as my concern in this post was to elucidate the genetic logic clearly and I believed that that logic would be muddled if I let stand the sociologically constructed dichotomy of a “black” and a “white” twin.

Comments

  1. #1 Julia
    October 22, 2006

    I much appreciate your posts; they’re almost always both interesting and informative.

    One small objection here: It doesn’t seem to be very kind to refer to “the pallor of someone of European descent” as “pallor” is generally a negative term suggesting a whiteness that is unnatural and/or unhealthy.

  2. #2 JP
    October 22, 2006

    excellent point, julia. why the hate, razib? I’m ashamed of you. Have you no reverence for fine lawn sleeves?

  3. #3 dilettante
    October 23, 2006

    First, the “black” twin is clearly mixed race. Is she? I previously would have agreed -
    but having met some ‘pure’ Africans who would be offended at the suggestion of miscegenation in their family lines- I’m not so sure that you can make statements like that.
    Also her skin tone matchs many other ~Afro Americans who had a non black grand/great grand parent on both sides.
    [dude- were you the protaganist in Hari Kunzru's The Impresionist--well maybe a basis for the character]

  4. #4 razib
    October 23, 2006

    but having met some ‘pure’ Africans who would be offended at the suggestion of miscegenation in their family lines

    whether they would be offended is pretty irrelevant isn’t it? there are people in southern africa who have no european admixture who are rather light, but that is because southern africans are naturally lighter (and the khoisan are often medium-light brown). but that is generally not the case in west africa, the source population for jamaica.

  5. #5 razib
    October 23, 2006

    p.s. everyone view the skin color map please. puts in context what i’m trying to say….

  6. #6 Dilettante
    October 23, 2006

    I don’t think the suggestion of miscegenation being offensive is irrelevant. Most Caribbean’s and some Afro Americans– of ‘mixed ancestry’ are quite anxious to inform you know that they are not ‘all black’. There are brown skinned people of ‘pure’ African descent- who have not mixed with another racial group. Their [African's] being offended at the suggestion highlights a different mind set regarding skin colour in my view.

    If you didn’t know from her interviews would you see Halle Barry as ‘clearly mixed race’ ? How about Vanessa Willams (Miss America) or her runner up Suzette Charles. Lena Horne?

    I understand the context of your post but the map you point to is not useful in referring to -’black’ populations 12+ generations removed from Africa.
    (so what’s up w/ you and Hari Kunzru?) ; )

  7. #7 razib
    October 23, 2006

    If you didn’t know from her interviews would you see Halle Barry as ‘clearly mixed race’ ? How about Vanessa Willams (Miss America) or her runner up Suzette Charles. Lena Horne?

    genetically & phenotypically of course they exhibit clear admixture (many african americans were defined as ‘Colored’ [mixed-race] in social situations in south africa precisely because that society recognizes this grouping). sociologically i understand though that ‘black’ is governed by hypodescent, though i tend to defer to personal self-identification in a case-by-case basis. but, it isn’t hard to identify on a pacific northwest college campus international students from africa as opposed to local black americans, the latter exhibit a lot of white admixture which stands out next to individuals from ghana, liberia, nigeria, etc.. the fact that jerry rawlings is half-west african and half-scottish makes him “black” in the USA, but genetically he is mixed, and i would not be surprised if in west africa he was taunted as ‘white’ as a child.

    but in any case, i’m a little confused as to your line of questioning. we know for a fact that the ‘black’ girl is mixed-race, her father is white, and her mother has a white parent, so genetically she is at least 3/4 white. admixture testing shows that jamaican “blacks” are about 0.9 west african (vs. 0.1 european). my own personal experience which people of unadmixed coastal west africa (as opposed to fulani, for example) descent is that such a complexion as the baby above has would be somewhat atypical (thought not impossible), so the conditional probabilities in my estimation lean toward her color being due to a synthesis between european and african identical by descent alleles. remember, we know for a fact that half of her skin color alleles are european by ancestry, so it seems highly unlikely that unless this girl is being put through tanning that her west african ancestors passed to her mother atypical (lighter) skin color alleles for their population.

  8. #8 razib
    October 23, 2006

    haven’t seen the movie btw.

  9. #9 Mouth of the Yellow River
    October 23, 2006

    Ni Hao! Kannichi Wa!

    Great graphic.

    Which twin would have suffered the most from historic American one-drop rule and Jim Crow laws? Who of the two will suffer on average the most from today’s modern versions of one-drop and Jim Crow?

    Surprise, the twin on the right in both periods. See Essays on the Color Line and the One-Drop Rule.

    MOTYR

  10. #10 Ruchira Paul
    October 23, 2006

    Completely off topic here. As a self professed aelurophile, should you be using expressions like many ways to skin the cat?

  11. #11 razib
    October 23, 2006

    there’s a back story…i would tell you to ask her, but she’s not fluent in english….

  12. #12 Pearsall
    October 24, 2006

    If the child on the left was born in Jamaica I suspect that based on phenotype the population would classify it as “mixed-race,” not black.

    Browning is the Jamaican term.

  13. #13 chiz
    October 26, 2006

    Heteropaternity is another possibility although I suspect that if this in fact the case the mother wouldn’t admit it:-)

    Everyone always forgets about heteropaternal twinning -especially the people doing twin studies – even though we’ve known about it for centuries, and it probably isn’t rare.

  14. #14 razib
    October 26, 2006

    Everyone always forgets about heteropaternal twinning -especially the people doing twin studies – even though we’ve known about it for centuries, and it probably isn’t rare.

    here is how rare:

    Sometimes superfecundation occurs by two different men. The frequency with which this occurs must depend on rates of infidelity (promiscuity). It is suggested that among DZ twins born to married white women in the U.S., about one pair in 400 is bipaternal. The incidence may be substantially higher in small selected groups of dizygotic twin maternities, eg. those of women engaged in prostitution.

    of course, there aren’t that many black or mixed-race males in australia.

  15. #15 plucky punk
    October 31, 2006

    Interesting post. I’m mixed race, and my husband is of European descent. My family is kind of surprised at how “white” our daughter is. My mother, who is Italian-American, was kind of sad growing up to have a child who really didn’t look like her, and I think now she’s worried that I’ll have the same sort of problems with people talking, etc.

    Personally, I don’t think any of it matters, but what do I know. It’s difficult to be mixed-race in the United States, as it is always tied up in some sort of racial pride thing. Like, if I don’t just call myself black, then I’m ashamed of being black, no matter how Italian I might also be.

  16. #16 razib
    October 31, 2006

    cute baby.

  17. #17 Mixed-Race
    October 31, 2006

    The way Razib was talking about genes being “on” or “off” suggests that the “on” allele should be dominant to the “off” allele. But that would mean that a child born to a mixed-race mother and a white father could never be darker than the mother. Yet it does happen, unless you conclude that all such cases are really cases of false paternity.

    I am mixed-race myself and quite light. There seem to be a lot of stories in African American culture of women like me who marry white men and end up with (Surprise!) a very dark little baby, which the man then proceeds to reject, and she has to return heartbroken to her own people. I’ve always thought that such stories were mostly apocryphal, meant to remind us not to reject our heritage, and the actual phenomenon was much less common than you’d think from the stories. But according to your reasoning, it should not be able to happen at all.

    Is it just that you oversimplified the argument for the sake of clarity? I don’t know much about the genetics of skin color, but I think there are more than two alleles at each locus, and not all alleles penetrate equally well.

  18. #18 razib
    October 31, 2006

    Is it just that you oversimplified the argument for the sake of clarity? I don’t know much about the genetics of skin color, but I think there are more than two alleles at each locus, and not all alleles penetrate equally well.

    1) yes, i oversimplified

    2) yes, there are more than two alleles per locus, but the reality is that the outcome is what matters, in for example east asian alleles and european alleles which differ result in the same loss of function outcome

    3) no, dominance is not really happening above since i stipulated additivity. that is, of both copies are “on” you are twice as productive as if only one copy is “on.” so, on & off is equidistant between on && on and off && off.

    4) not all the variance in skin color is due genotype, there is development and environment, etc., so other factors could at be at work

    5) there maybe gene-gene interactions that result in a non-linearity, or unpredictability, to the outcome from the genetic combinations in some cases, as well as dominance, etc.

    nevertheless, the model above seems pretty good for the vast majority of cases. it was formulated first in the 1960s using mixed-race communities in england, and sequencing data today is confirming around 5 primary alleles with approximate additivity.

  19. #19 razib
    October 31, 2006

    and to be clear,

    medium mother + light father => darker than mother baby

    should be impossible from the model above, but this is a first approximation, there are many ways to genetically and biologically imagine how this would happen (gene-gene interactions are the primary one, as the combination results in novel expression patters). nevertheless, the vast majority of the cases will be like the children above rather than this pattern.

  20. #20 NuSapiens
    November 2, 2006

    Check out the movie “Tsotsi” to see the skin tone range in South African blacks. I have no idea whether the actors are mixed (probably not much), but the lead actor has interesting light eyes (some unusual shade that isn’t quite brown or blue as far as I can tell). Some of the other actors/actresses also have features and skin tone that would be called “medium” in the USA and attributed to white mixture.

  21. #21 razib
    November 2, 2006

    prolly “unmixed.” quotes because khoisan admixture is common among groups like the xhosa (look at nelson mandela, and he is from a princely lineage), and khoisan are more brownish than dark brown.

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