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Health

Twins of different races, 2009

Gene ExpressionBy Razib KhanJanuary 2, 2009 12:53 PM
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Every now and then British newspapers publish the story of twins born to an interracial couple who seem to resemble only one of the races of their parents. The irony being that siblings may appear to be of different populations. I've commented on several of these stories before. Now we have another one to ring in the new year, Mixed-race couple give birth to black and white twins - for the second time:

Seven years after having one black twin and one white twin, the 27-year-old mother has given birth to a second set of twins with different coloured skin at odds of one in 500,000. When the couple's first daughters arrived in 2001, they were astonished to see that Lauren had her mother's blue eyes and red hair, while her twin Hayleigh had dark skin and hair, like her father, Dean. So when Miss Spooner, from Fleet, in Hampshire, found out she was pregnant again this year, her friends and family joked that they ought to take a bet out on the same thing happening again. Doctors who delivered the sisters early, because of fears for their health, were relieved to find them well, but also amazed that the second set of twin girls were born with different skin tones. As her parents discovered when the girls were laid side by side in the hospital cot, Miya resembles her father and Leah has inherited her mother's looks. It is rare enough for two sets of twins to be born to the same parents, but the chances of them inheriting different skin and hair colour from their mother and father are just two in a million.

I would take issue with several points in the article: 1) The "black" father and daughters are of mixed appearance. In fact, by this photo the complexions of the black father and daughters are far lighter than the typical person of West African ancestry (the father is West Indian). It seems certain that the father is of mixed European and African ancestry. His "black" daughters exhibit features of this mixed ancestry. In contrast, his white daughters do not, having inherited genetic variants of a European nature from their mixed-race father. 2) There are several genes where the variation between Europeans and Africans is disjoint. In other words, almost every person of African and European ancestry have the same variant within population, and different between. One example of this is SLC24A5. It is possible that the father is a heterozygote, carrying one variant for light skin, and one variant for dark skin. In that case, each one of his children has a 50% chance of carrying the light skin variant, and a 50% chance of carrying the dark skin variant. Since the mother is European, the potential combinations on this locus would be heterozygote, or homozygote for the light skin variant. Expand this process to 3-4 other loci, and you see that the variance within the sampling process means that the differences you see among these siblings are not surprising. 3) Twinning runs in families. I suspect that the probability generated above does not take into account the fact that someone who has twins naturally once is more likely than the norm to have twins in the future, because the fact of having twins initially suggests a greater likelihood that the individual may produce twins. On other words, I doubt the 2 in 1 million number takes into account conditional probabilities....

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