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Razib Khan
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0879696753 Several people have contacted me about the Aylmer twins, who exhibit very distinct phenotypes. In short, one twin is very fair skinned, to the point of being a redhead, while the other twin has visibly African features and a darker complexion. What caught my attention is that their surname is the same as the middle name of R. A. Fisher. Perhaps they are related in some fashion to the father of evolutionary genetics?

The “science” behind the story is not particularly novel. Rather, for whatever reason the British tabloid press in particular seems to love publicizing variation in phenotypes which are racially coded in mixed-race families. I’ve been talking about “black and white twins” for so long that it’s not something I really wanted to revisit, as there isn’t much novel to say.

But, it is important to note that the media representations of this phenomenon often play a semantic shell game. Though some in the press are not reporting it, is not hard to find out that the black parent in this instance, the twins’ mother, is actually mixed-race. That is, she is about half African in her ancestry. Therefore, though the expected proportion of African ancestry in her daughters is 25 percent, random variation could result in individuals with sharply increased, or decreased fraction. People of mixed African and European heritage who exhibit visible African ancestry are often coded socially as black. But these twins are not “biracial” in a symmetric sense when it comes to genetics, even if they are as it concerns the social construction of racial identity.

One way the science here could be demystified is taking into consideration this important genealogical detail of the mother and making an analogy to populations. The twins can be thought of as backcrosses, as their mother is an F1, and the father is a “parental” type (100 percent European). In a genetic trait which is governed by one gene crosses back to the parent population of an F1 can result in a wide range of phenotypic outcomes, depending on the nature of the expression of the alleles. Since human pigmentation is governed by a relatively small number of genes (most of the inter-population variation is probably due to half a dozen loci), some of the same dynamics applicable to monogenic traits in the case of a backcross apply here, though to a diminishing extent. As it turns out in this case the “black” twin invariably has the traits of both African and European populations, just like one of their parents, the F1, while the white twin resembles one of the “parental” populations.

Razib Khan
About Razib Khan

"I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. If you want to know more, see the links at"