You can also see in the bar chart that compared to the European ancestry component, there is much less variation in the different African subgroup components of ancestry -- in other words, there aren't some individuals who have much more Yoruban ancestry and others that have much more Bantu ancestry. This is why we concluded that it is likely that mating patterns in African Americans probably did not strongly reflect actual origins in Africa. And that is also the reason we concluded that because most African Americans appear to have admixed African ancestry, looking only at a single genetic location (e.g. the Y chromosome or mtDNA, as often done by ancestry companies) gives only a narrow picture of the entire ancestry.
Subsequent figures in the paper pretty much reinforce these conclusions.
OK, so that helps. A lot. Here is another question. I want to know what someone with your background thinks about the notion of "race." As a writer, I approach this through the lens of history. I imagine, because of that, I might be missing some things. I want to know, as a geneticist, whether you think of African Americans as a "race?"
I believe it is inaccurate to refer to African Americans as a race or racial group (much as it is similarly inappropriate to refer to Latinos that way) -- unless you move away from the more classical definitions of race. We try to use the term race/ethnicity. There has been a lot of debate about whether genetic variation in the human population is continuous or discrete. From my view, it is both. This is what makes it challenging to create categories.
One question pops out at me. You indicate some suspicion to referring to African-Americans as a "race" but (in some of your research) you support using "race" in terms of collecting med data and disease studies. Is this a case of a definition -- though it may be imperfect, clunky and at times even misleading -- still telling us something? From what I gathered from those articles "race" can be a proxy not just for genetic stuff, but for social phenomenon too (such as access to health care.) Am I seeing that right? Is it correct to say, for instance, "Yes, race is a social construct, but this does not make it meaningless." It still useful to look at "race," for instance, when studying sickle-cell. Perhaps some day, when we have more refined technique, it won't be.
Definitions can indeed be "clunky." I would use the phrase race/ethnicity rather than just race because in common parlance it is a better description. I tend to think that race has been used more in terms of continental origins (Africa, East Asia, Europe, Americas). On that basis, one would not characterize African Americans as a racial group, but rather as an ethnic group. We sort of implied this in the Genome Biology paper. The reason is that African Americans typically have European as well as African ancestry (and possibly other ancestries as well) and are also culturally distinct from Africans. Sort of similar to Latinos - who from a genetic ancestry standpoint can be nearly anything. Hence our use of race/ethnicity.
Just to opine a bit, I think part of the problem is the notion of a causal relationship -- i.e. "dark-skin" or "blackness" causing sickle-cell -- as opposed to a more geographic definition that might encompass people regardless of skin color.
Yes, exactly. Groups living in isolation from each other for long periods of time have acquired many genetic differences. The large majority of those are due to "genetic drift" -- i.e. random fluctuations in gene frequencies. That also includes many genetic variants that code for traits and diseases. But then there are some genetic variants that differ in frequency due to differential selection pressure in different environments. The best examples are for genes that confer resistance to malaria. One of those causes sickle cell disease in those who carry two mutations; those who carry one copy have sickle cell trait, which is generally benign but confers greater resistance to severe malaria infection. Mutations for sickle cell disease are found at pretty high frequency in some African populations, but also found in parts of the middle east and India. Beta thalassemia is another disease where carriers are offered greater protection from malaria. This disease is more common around the Mediterranean (e.g. Greeks).
Then there is G6PD deficiency. Mutations for that are found at increased frequency in parts of Africa, but also in the Middle East. The mutations underlying these disorders generally differ geographically, which is another indication that while the mutations are different ancestrally, they achieved high frequency in different populations for similar reasons (i.e. resistance to malaria). Another more recent example is a gene called ApoL1. There are a couple of genetic variants found in West Africans (and African Americans); when carrying two of these, there is an increased risk for kidney disease if hypertensive. It was shown that these variants likely provide some immunity from African Sleeping Sickness (tsetse fly disease) which may have led to them becoming more common where the disease is prevalent.
Various populations have an increased frequency of genetic diseases, which are often unique. Probably a lot or most of it is just chance, but perhaps not all of it. Proving historical selective advantages can be pretty challenging. So, as I mentioned above, groups living in isolation developed their own genetic (and cultural) profiles. Generally, there is no cause and effect between the traits that differentiate groups. East Asians have dark hair and eat with chopsticks. But there is no causal relationship. You can use a whole variety of different traits to place individuals into the same categories, but those traits may have nothing to do with each other etiologically.