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November 12, 2013

Is that white supremacist part black?

Filed under: Craig Cobb,race — Razib Khan @ 9:45 am

Goebbels, Nordic superman! Credit: Bundesarchiv, Bild

Goebbels, Nordic superman! 
Credit:, Bundesarchiv, Bild

Many of you probably know about Dave Chappelle’s black white supremacist sketch (NSFW video!), though fewer are aware of Leo Felton, a white supremacist (ex, after he was outed) with a black father (a less tragic outcome than Dan Burros, the Jewish American Nazi). I know, these sound like they’re out of South Park episodes, though the last two are actually not fictional. But now the media is exploding with news that a DNA test has suggested that a notorious white supremacist is 14 percent black. The same one who was recently profiled in The New York Times promoting a separatist racial vision in a small North Dakota town. I’ll be honest and admit that I don’t think that these results will hold up. (though personally I would think it was rich and very funny if they did, just like everyone else).

Credit: ScottH

Credit: ScottH

The reason is the chart to the left. It’s from 23andMe‘s data set. Out of their ~100,000 white American individuals tested, ~5% have any evidence of African ancestry. Of those, you see the distribution of results. If Craig Cobb, the white supremacist, is ~14% Sub-Saharan African, he’s in the less than 0.1% of white Americans with this sort of pattern. If he was a Latin American white, or a identified white person of Arab ancestry, I’d be willing to accept the results as plausible on the face of it. But the reality is that European Americans with relatively well documented histories usually do not have a high probability of having African ancestry. And if they do, 14% is a great deal. I have seen this among my friends (or more honestly, 5-10%, which is not far off), but that was due to a cryptic (though somewhat known within the family) non-paternity event.

The media isn’t consistent about which firm tested Craig Cobb, so I’m not going to make accusations specifically, but he says he’s getting other tests done, and he’ll release the results. I’ll be curious to see the raw results. To me this is reminiscent of the constant Facebook shares I get from the Daily Currant from friends who confuse satire for reality because of their biases (not to say those biases are unfounded or not). Just because the story is awesome does not mean we should set skepticism to the side, and not evaluate the probability of alternative outcomes, given what we know prior.

Related: Has anyone followed up the old story that James Watson was part black? The timing was a little suspicious to me.

The post Is that white supremacist part black? appeared first on Gene Expression.

September 15, 2013

Miss America, Nina Davuluri, does not look like Miss India

Filed under: Beauty,Miss America,Nina Davuluri,race,Racism — Razib Khan @ 11:40 pm
Miss America 2013 is an Indian American woman, Nina Davuluri. This has predictably ushered in lots of sad and sometimes so-sad-it’s-funny (frankly) racism on Twitter. But there’s another interesting angle: a friend pointed out that Nina Davuluri is probably too … Continue reading

August 2, 2013

Gendered perceptions of parental identity

Filed under: race,Sexism — Razib Khan @ 11:32 pm
At the super market my wife pointed out an article in the parenting section which she stumbled upon while waiting for me to finish at the checkout line, Is She Yours? In a stray moment I decided to see if … Continue reading

August 9, 2012

The law of reversion to type as cultural illusion

Filed under: Admixture,Genetics,race — Razib Khan @ 12:01 am

A comment below:

Does the higher genetic diversity in sub-Saharan Africans explain why mixed children of blacks + other couples usually look more black than anything?

As in, the higher number of genetic characteristics overwhelms those of the other parent and allows them to be present in the child.

But this makes you ask: is the assumption that people with some African heritage tend to exhibit that heritage disproportionately even true? From an American perspective the answer is obviously yes. But from a non-American perspective not always. Why? Doe the laws of genetics operate differently for Americans and non-Americans? I doubt t. Rather, hypodescent, and its undergirding principle of the “reversion to the primitive type” are still background assumptions of American culture. In fact today black Americans are perhaps most aggressive and explicit in outlining the logic and implications of the “one drop rule,” though non-blacks tend to accept it as an operative principle as well.

Assessing someone’s racial identity has a subjective aspect. We see through the mirror darkly, and that’s a function of the cultural preconditions of gestalt cognition. But there are some objective metrics we can look ...

July 25, 2012

A little food & medicine go a long way

Filed under: Caste,Lower Caste,Prejudice,race — Razib Khan @ 7:45 am

I clicked through on the links to Zack’s post below and was pretty shocked. I know this somewhat, but not having grown up around many Indians (or South Asians generally) I didn’t have a good sense. That being said, a few years ago I stumbled on a book at the local book store, Daughter of the Ganges: A Memoir. The author was an adopted woman of Indian heritage from Spain. Skimming through, her family seems to have been peasant cultivators in Maharashtra. Therefore, I was struck by her photo. She is rather attractive and not “worn down” by the life of extreme subsistence.

This is not to take anything away from Zack’s post, and she obviously does not look like the typical upper caste NRI even to my unsubtle eyes. But a great deal of the physical difference in terms of perception is environmental. Though I do think it is telling that a woman who looks like this could never be a leading lady in a Bollywood film.

Asha Miro


July 22, 2012

The Scots-Irish as indigenous people

Filed under: Culture,race,Scots-Irish — Razib Khan @ 4:04 pm

A fascinating comment below:

In traveling across America, the Scots Irish have consistently blown my mind as far and away the most persistent and unchanging regional subculture in the country. Their family structures, religion and politics, and social lives all remain unchanged compared to the wholesale abandonment of tradition that’s occurred nearly everywhere else.

Unfortunately, this has a lot to do with a powerful and long-running strand of paranoia and xenophobia. I’ve ridden trains through the rail towns of WV and KY and been regarded with more unprovoked hatred than anywhere else on Earth. On the other hand, when I’ve been introduced to their clan-based social structures by close friends, it is a uniquely close-knit and life-affirming culture that I’ve been honored to participate in.

What stuck me about this comment is that it is the sort of statement you regularly see from Western anthropologists or adventure tourists in relation to indigenous colored peoples the world over. That is, a parochial clannish folk trying to hold onto to their traditions, albeit with the downside of being inward looking and often regressive (downside from the perspective of Westerners that is). What these people ...

May 25, 2012

Fear of a black past

Filed under: Melungeon,race — Razib Khan @ 9:35 pm

I notice that the media has started reporting that scientific genealogy has now established to a great extent the likely origin of the Melungeons. You can find the original paper online. The gist is that the Melungeons seem to exhibit a large proportion of Sub-Saharan African origin Y chromosomal lineages, and European mtDNA lineages. The lack of Amerindian ancestry in the generality is also notable. But, this does not entail that the origins of the Melungeons is from the union of free black males and white women necessarily, at least on purely genetic grounds (the paper itself has a wealth of genealogical evidence pointing to this likelihood). The Melungeons are an endogamous community, and so have a low effective population. African or Amerindian mtDNA lineages may simply have been lost by chance over the past few hundred years.

But I point to the story of the Melungeons because it is a nice counter-point to that of the Hispanos of the Southwest. This is a case where historians and anthropologists who made the case for the false construction of a mythical Middle Eastern ancestry for the Melungeons as a way in which to escape the bounds of the ...

May 20, 2012

Education encourages integration?

Filed under: data,Demographics,race — Razib Khan @ 10:36 am

It is sometimes fashionable to assert that higher socioeconomic status whites are the sort who will impose integration on lower socioeconomic status whites, all the while sequestering themselves away. I assumed this was a rough reflection of reality. But after looking at the General Social Survey I am not sure that this chestnut of cynical wisdom has a basis in fact. Below are the proportions of non-Hispanic whites who have had a black friend or acquaintance over for dinner recently by educational attainment:

35% – Less than high school
36% – High school
47% – Junior College
45% – Bachelor
59% – Graduate

I thought this might have been a fluke, so I played around with the GSS’s multiple regression feature, using a logistic model. To my surprise socioeconomic status was positively associated with having a black person over for dinner, and age negatively associated. These two variables in fact tended to exhibit equal magnitude values in opposition, and always remained statistically significant. Just to clear, I created a variable Non-South vs. South below (being Southern increases likelihood of having had a black person over for dinner). All the individuals surveyed are non-Hispanic whites for the year 2000 and ...

May 4, 2012

Richard Dawkins accepts the usefulness of race

Filed under: race,Richard Dawkins — Razib Khan @ 10:57 am

There have been a variety of responses to my column in The Crux on race. To be fair, because the audience for The Crux does not consist of genome nerds I engaged in some first approximations which some readers have taken objection to. For example, the genetic architecture of blue vs. brown eye inheritance is ‘quasi-Mendelian,’ with ~75 percent of the variation in Europeans on this trait attributable to variation in the region of the HERC2 and OCA2 genes. But I thought, and still think, that a rough re-characterization of the trait as a recessive one with a monogenic Mendelian inheritance pattern can be justified for didactic purposes (just like one can justify the idea that whole human genomes have been sequenced, even if there are large gaps, and known errors in regions of repeats).

But the comments over at Richard Dawkins’ website have been rather amusing as a whole. Some people were patronizingly pedantic, and I don’t have time respond in detail (the real response would be: read my blog!). But some comments are easy to answer:

I’d like to hear Dawkins’ response to this, I’ve read all of his books and he’d surely disagree with ...

April 23, 2012

An algorithm is just an algorithm

Filed under: Anthroplogy,race — Razib Khan @ 8:48 pm

In the comments below:

You should include a Moroccan or otherwise native North African sample. Without a North African sample West Africans act as proxy for some of that North African ancestry that does exist in Iberia, specially the Western third (Portugal, Galicia, Extremadura, León, etc.) Doing that your analysis would become more precise and you could make better informed claims.

I was reading through all the entry and there was no mention to the rather surprising notable West African component in Iberians other than Basques. For my somewhat trained eye it is clear that this is a proxy for North African ancestry and not directly West African ancestry. This is demonstratedly also the case in Canary Islands, at least to a large extent, and, by extension in Cuba (which is nearly identical to your average Canarian), at least Cuba-1. Cuba-2 seems actually admixed at low levels and both seem to have some Amerindian ancestry not existent in Spain.

This is a fair point. I switched computers recently, and the Behar et al. data set I had seems to have become corrupted. So I snatched the Mozabites from the HGDP, and removed the Gujaratis from the previous run. I also added Russians, ...

April 22, 2012

A deeper dive analysis of two Cubans

Filed under: Anthroplogy,race — Razib Khan @ 10:42 pm

About a week ago I put up a post put on an analysis of a paper which reported on the ancestral make up of 50 Cubans (as well as assorted other Hispanic/Latino groups). One aspect of the paper which was somewhat notable is that 1 out of 3 Cubans were 90 percent or more European in ancestry. The notability of this is that is that 5 out of 6 Cuban Americans identify as white. That is, of European ancestry. The main caveat here is that these Cubans were sampled from New York City, and to a lesser extent the Midwest. The fact of non-European admixture in putatively white European individuals from Latin America is not surprising. Our prior expectation should be that the admixture is non-trivial, though not preponderant. For example, the majority of the white population of Argentina has Amerindian ancestry (or, more precisely ~15 percent of the aggregate ancestry of Argentineans is Amerindian). At least notionally Cuba is a much more racially mixed culture than Argentina, so non-white admixture in even white Cubans is not surprising.

Based on the above paper (and the data which you can find on other Latin American whites), as well as ...

April 15, 2012

The case of the white Cubans

Filed under: Anthroplogy,Human Genomics,race,white Hispanics — Razib Khan @ 5:11 pm

In a follow up to a post below, a new paper in PLoS Genetics has some data on American Hispanics. Specifically, Puerto Ricans, Dominicans, Mexicans, and Cubans, as well as assorted Central and South Americans. I am not too interested in the cases except Cubans; no one doubts the mixed heritage of the other groups, though the African ancestry of Mexicans, and some Central and South Americans may surprise (again, I have to note that this not surprising in light of history, and has been robustly confirmed in the genomic literature).

But Cuban Americans are somewhat a special case. The vast majority, specifically, 85 percent, identify as white. This is a higher proportion than the number of self-identified whites in Cuba, and a function of the skewed nature of the migration out of Cuba socially and economically. By and large the white elite of the island fled Castro’s revolution to a far greater extent than the black lower classes. And contrary to American stereotypes of Latin American ease and openness about race, Cuba was a relatively stratified society, albeit not characterized by hypodescent. Slavery was ...

March 7, 2012

Where the wild clines aren’t

Filed under: Anthroplogy,Human Genetics,Human Genomics,Population genetics,race — Razib Khan @ 7:38 pm

In the recent ‘do human races’ exist controversy Nick Matzke’s post Continuous geographic structure is real, “discrete races” aren’t has become something of a touchstone (perhaps a post like Cosma Shalizi’s on I.Q. and heritability).* In the post Matzke emphasized the idea of clines, roughly a continuous gradient of genetic change over space. Fair enough. But in the map above I traced two linear transects. I would suggest that anyone who has a general understanding of the demographics of South-Central Eurasia would immediately anticipate that these transects would reveal a relatively sharp break in allele frequencies. True, there are intermediate populations between the two end points, in Nepal, and on the fringes of India’s northeastern states. But clearly about halfway through the southwest-northeast transect you’ll see a rapid shift in allele frequencies. The blue transect is different, insofar as the change occurs very near its eastern pole. In Bengal, 85% of the length of the transect from its western terminus, the populations will still be far closer genetically to those on the western pole than those just to the east!


I thought of this when I saw that Zack had posted a Tibetan data set from Qinghai. As the crow flies Qinghai is closer to the plains of North India than peninsular South India, but Zack found Tibetans from this region to be only ~1 percent South Asian. That’s likely to be close to noise. I assume this does not surprise anyone. Despite the fact that North India is very populous in relation to Tibet, it turns out that geographical barriers are very strong in discouraging gene flow (note that Tibet and North India are actually culturally related; Tibetan Buddhism has its origins in the Tantric Buddhism of Bengal). This is one of my major “beefs” with the idea that “race does not exist” because of clines. I think this is a robust point when it comes to there being no Middle Eastern race vs. Scandinavian race. The clines are real and gradual between these two population sets. But I do think there has been strong differentiation between populations from the antipodes of Eurasia. I suspect that the emergence of more flexible lifestyles (e.g., oasis agriculture, horse nomadism) has in fact resulted in far greater connections between the isolated zones of Western and Eastern Eurasia over the past 10,000 years than before. In fact, one can conceptualize it as a two fold process. On the one hand you had very powerful expansions from small initial founder groups across macro-regions such as Western Eurasia and the Far East. This resulted in a decrease of genetic difference within these zones through the power of homogenization, though increased Fst in the few zones of direct contact across the zones. But, the “empty zones” of Central Eurasia may also have filled up with”proto-”Silk Road” centers over the past ~10,000 years, resulting in more frequent long term connections between the macro-regions than had heretofore been possible.

* I guess I should divulge that I have socialized with Nick Matzke an that we share common friends.

March 2, 2012

Race: maybe it’s agriculture

Filed under: Human Genetics,race — Razib Khan @ 3:23 pm

I’m too busy to really blog today, but I thought of putting up a post, the gist of which was actually expressed in Ian’s comment below:

When I was younger, I thought of human races as archetypes, and the variation between them a product of mixing. I blame it on the fact that I read Coon when I was about 14. Still, as a (half)Indian, it’s hard to see reconcile the reality of a billion people in the subcontinent with models that try to classify people into 3-5 races. As I learned more biology, I came to the conclusion that human variation was clinal, and race was really an artefact of where you chose to sample along the continuum…as a plant ecologist, I think about things like that a lot. (I’m also somewhat skeptical of ecozones.)

Thanks to a number of convergent strands (of which Razib’s blogging has been a key element), I have come to a rather different conclusion. Race, in my opinion, is more a feature of agriculture than evolution.

Consider two possible models of race: Model 1, in which sharp distinctions existed before the Neolithic, and have been maintained and enhanced as certain groups adopted agriculture and displaced their hunter-gatherer neighbours; and Model 2, in which variation was clinal prior to the Neolithic, but that the immense demographic expansion of certain groups expanded THEIR specific points on the continuum, and brought them into contact (or nearly into contact) with other expansionist agriculturalists.

To me, the Model 2 seems more plausible than Model 1. Is that an argument against race? No, but it does suggest that races shouldn’t really be seen as “locally adapted optima” and rather, should be seen more as transient phenomena produced by historic contingency. Whether this means that race is “real” or not is, to me, a little beside the point. But I’m not convinced by Coyne’s argument that these differences represent the “accumulation of genetic differences between isolated populations”.

A hybrid?

Isolation-by-distance is a powerful null model. But if Reconstructing Indian History is correct then it is a poor description of the recent history of the Indian subcontinent. Since there are ~1.5 billion South Asians I think that that is a major objection. And I don’t think it is just South Asians. Even 10,000 years ago I suspect that a clinal isolation-by-distance model would be confronted by several clusters of demes which were operationally allopatric (e.g., West Eurasia & North Africa vs. East Eurasia, Austarlasia, the New World, Sub-Saharan Africa, and likely South Asia). This probably had to do with the fact that human occupation is not, and was not, evenly distributed.

Image credit: Wikipedia

February 29, 2012

Jerry Coyne on race: a reflection of evolution

Filed under: Human Evolution,race — Razib Khan @ 8:53 am

After my post on the ‘race question’ I thought it would be useful to point to Jerry Coyne’s ‘Are there human races’?. The utility is that Coyne’s book Speciation strongly shaped my own perceptions. I knew the empirical reality of clustering before I read that book, but the analogy with “species concept” debates was only striking after becoming more familiar with that literature. Coyne’s post was triggered by a review of Race?: Debunking a Scientific Myth and Race and the Genetic Revolution: Science, Myth, and Culture. He terms the review tendentious, and I generally agree.

In the early 20th century Western intellectuals of all political stripes understood what biology told us about human taxonomy. In short, human races were different, and the white European race was superior on the metrics which mattered (this was even true of Left-Socialist intellectuals such as H. G. Wells and Jack London). In the early 21st century Western intellectuals of all political stripes understand what biology teaches us about human taxonomy. Human races are basically the same, and for all practical purposes identical, and equal on measures which matter (again, to Western intellectuals). As Coyne alludes to in his post these are both ideologically driven positions. One of the main reasons that I shy away from modern liberalism is a strong commitment to interchangeability and identity across all individuals and populations as a matter of fact, rather than equality as a matter of legal commitment. In a minimal government scenario the details of human variation are not of particular relevance, but if you accept the feasibility of social engineering (a term I am not using in an insulting sense, but in a descriptive one) you have to start out with a model of human nature. So this is not just an abstract issue. For whatever reason many moderns, both liberals and economic conservatives, start out with one of near identity (e.g., H. economicus in economics).

I want to highlight a few sections of Coyne’s post:

What are races?

In my own field of evolutionary biology, races of animals (also called “subspecies” or “ecotypes”) are morphologically distinguishable populations that live in allopatry (i.e. are geographically separated).  There is no firm criterion on how much morphological difference it takes to delimit a race.  Races of mice, for example, are described solely on the basis of difference in coat color, which could involve only one or two genes.

Under that criterion, are there human races?

Yes.  As we all know, there are morphologically different groups of people who live in different areas, though those differences are blurring due to recent innovations in transportation that have led to more admixture between human groups.

Why do these differences exist?

The short answer is, of course, evolution.  The groups exist because human populations have an evolutionary history, and, like different species themselves, that ancestry leads to clustering and branching, though humans have a lot of genetic interchange between the branches!

But what evolutionary forces caused the differentiation?  It’s undoubtedly a combination of natural selection (especially for the morphological traits) and genetic drift, which will both lead to the accumulation of genetic differences between isolated populations.  What I want to emphasize is that even for the morphological differences between human “races,” we have virtually no understanding of how evolution produced them.  It’s pretty likely that skin pigmentation resulted from natural selection operating differently in different places, but even there we’re not sure why (the classic story involved selection for protection against melanoma-inducing sunlight in lower latitudes, and selection for lighter pigmentation at higher latitudes to allow production of vitamin D in the skin; but this has been called into question by some workers).

As for things like differences in hair texture, eye shape, and nose shape, we have no idea….

I have no idea if reading Coyne’s earlier work influenced me, but observe that he too emphasizes that human races are a reflection of evolutionary history. Some of my interlocutors believe it is essential to have a tree-like phylogeny with no reticulation (gene flow across branches) to have a reasonable model for race, but I do not. That’s because the focus for me is evolutionary history. I want to understand evolutionary history. Taxonomy is a means to that end. It is not the end.

Coyne has a follow up post which will be of no surprise to reader of this weblog. But I do want to add a few things. 1) For pigmentation we do now understand its genomics relatively well. It seems that light skin emerged at least twice at the two ends of Eurasia, and, that it was a recent emergence (as evidence by markers of selective sweeps). 2) As for hair texture, there is some work which has shed light on this. East Asians in particular carry a variant of EDAR which gives them their distinctive thick straight hair. There has been less work on “woolly hair,” but I suspect that it will be elucidated soon (there are some candidate genes, from linkage studies and animal models). Additionally, I think it is important to note that the dark-skin-as-protection-against-skin-cancer does not make much evolutionary sense. Melanoma strikes later in one’s reproductive years. Rather, I accept that Nina Jablonski has the right of it when she argues that it protects against neural tube defects which arise because of various chemical changes which occur in one’s biochemistry due to exposure to sun. Finally, I think Coyne underestimates the power of even gene genomics using haplotype based techniques in narrowing down on very specific geographical and population origins for segments of your DNA right now. The key is not where you come from, it is how segments of your DNA relate to the full range of segments of other peoples’ DNA.

February 23, 2012

The race question: are bonobos human?

Filed under: Genetics,Genomics,Human Genetics,Human Genomics,race — Razib Khan @ 1:05 am

Recently Jason Antrosio began a dialogue with readers of this weblog on the “race question.” More specifically, he asked that we peruse a 2009 review of the race question in the American Journal of Physical Anthropology. Additionally, he also pointed me to another 2009 paper in Genome Research, Non-Darwinian estimation: My ancestors, my genes’ ancestors. Normally I don’t react well to interactions anthropologists who are not Henry Harpending or John Hawks. But Dr. Antrosio engaged civilly, so I shall return the favor.

I did read all the papers in the American Journal of Physical Anthropology special issue, as well the Genome Research paper. My real interest here are specific questions of science, not history or social science. But I will address the latter areas rather quickly. I am not someone who comes to this totally naked of the history or social science of the race question. I’ve read many books on the topic. And as a colored person who has moderate experience with racism I get rather bored and irritated with excessively patronizing explanations of how racism afflicts us coloreds from white academics (non-white academics who focus on this subject are usually careerists or activists who don’t have to make much pretense toward scholarly substance and can be duly ignored, at least in my experience). The main point which I think we can all agree upon is that colloquial understanding of race has only a partial correlation with any genetic understanding of race. I myself have ranted against the confusions which have ensued because of the conflation of the two classes, and it is certainly a legitimate area of study, but it is not my primary concern. And importantly, I have no great primary interest in battling racism.


By this, I do not mean to imply that I support racism, or am personally against battling racism. When it comes to racists, broadly defined, I am not personally a great fan (as can be attested by my pattern of bans and rebukes). And when I say racism, I don’t just mean white people behaving badly. I mean people who express racial nationalist sentiments in a crude and crass manner, and are often inappropriately assertive about the righteousness of their views (e.g., a few commenters have complained that I, an Indian [yes, I'm not technically Indian], should not talk so much about Westerners. Of course I view myself a Westerner, but to a racialist this is simply not even wrong. Naturally this is a chasm in world-views which is not reconcilable. Please note that some “anti-racists” would also agree I am not a Westerner, though mostly because they view that term as referring to evil white colonialists). Nevertheless, when it comes a study of human variation, or history, and the like, my primary aim is to enter into a state of intellectual Epoché. Whether Charles Darwin, Francis Galton, or R. A. Fisher, were, or were not, racists is of minor concern to me. I am not saying that it is irrelevant, but the fixation on racial prejudice is not part of my bailiwick. As I allude to above there are whole departments devoted to the presumed oppression of coloreds by Mighty Whitey, and I leave them to their joyful intellectual romp.

But for many people ferreting out racism is more of a performative act. There’s a big difference between revealed preferences and avowed preferences. For example, most Americans espouse a love of diversity. But they sure don’t love diversity when it comes to who they date. These include many people who I know personally, who are diversity loving progressives, but who seem to fall into the trap of disaggregation. Since I don’t love diversity and don’t care about that issue I don’t bring it up with them often. But it’s what I call a revealed preference. Or, to give an amusing example, I said something offensive in one of my posts apparently a few years back, which prompted one outraged reader to leave a long shocked rant about my racism. The comment was trashed, and the reader banned. Nevertheless, I traced their Facebook account. The individual was a young white professional resident in San Francisco. And, their friends list was visible. I did a quick spot check, and estimated that ~90 percent of their San Francisco friends were white. In contrast, about ~50 percent of San Francisco’s population is white. I’m not going to accuse anyone of racism, but there are quite interesting revealed preferences in the world (I saw this when I lived in Berkeley, where a few times I was the only non-white at a party where people were trashing how little diversity there was in Oregon when they found out that that was where I was from). Most people like associate with “their own kind,” however that is defined.

That’s an observation. Not a judgement. I wish more people would withhold the judgement sometimes. Because I don’t really care about diversity and all the the standard shibboleths common among the progressive set, I do sometimes like to point out the naked emperors here and there. Frankly a lot of the humanistic and social science literature on race strikes me as performative as well. There are some nuggets of truth, but they’re usually trivially obvious. Segregation and genocide are generally agreed upon as bad. When the nuggets of truth are not trivial, they’re often strongly normative. Moral tales told, not positive descriptions of reality. I, for example, do not favor affirmative action, and do not care if academic departments reflect the racial diversity of society at large. This is not a common viewpoint in some circles. If you are on a university campus, I invite you to go look at the headshots of the graduate students in ecology & evolution, and then look at neuroscience. Count the number of Asians. You may see an interesting pattern!

So let’s move to the science. Do races exist in human biology? Is it a useful concept? That depends on criteria in both cases. The reality is that I’m not sure I know what a species is in an axiomatic sense, let alone race (many biologists don’t, that’s why there’s a whole area devoted to studying the issue of the definition). Rather, for me species are evaluated instrumentally. Is the classification of a set of individuals as a species useful in illuminating a specific biological question? Species are human constructions, categories which are mapped upon reality. That doesn’t make them without utility. Many of the same “where do you draw the line?” questions asked of race can be asked of species. In a deep ontological sense I don’t believe in species. But in a deep ontological sense I don’t accept the solidity of a brick (most of the volume is space of any object of course!).

Moving onto specific objections, some observe that genetic variation is clinal. This has a basis in fact, more or less. But the distribution of grades is also clinal. Nevertheless, professors generally look for “natural breaks,” and then distribute A’s, B’s, and C’s, accordingly. In concrete terms groups like the Tuareg and Uyghur are equidistant between West Eurasians and Africans and East Asians, respectively. But look at the map of the Old World’s population density. The variation in gene frequencies may be clinal, but that ignores the reality that the genetic clusters themselves have different weights varying as a function of space. The Tuareg are few. The “donor” populations on either side of the Sahara are many. If you want to look for “natural breaks,” you look to the empty spaces, where there will be populations, but very few.

Additionally, there is the question of history. We know that the Uyghur are a new population, which emerged in the past 2,000 years due to admixture between a resident West Eurasian population, and Turkic groups. We know this both through genetics (decay of linkage disequilibrium) and history. There is also a great deal of circumstantial evidence that the West Eurasian forebears of the Uyghurs, the Tocharians, were long distance migrants from the west. So who were the indigenes of the Tarim? It may be that due to the local ecology the center of Eurasia has long been relatively underpopulated in relation to the peripheries, with the emergence of new lifestyles (e.g., oasis agriculture, nomadism) resulting in the ethnogenesis of groups which arose recently to occupy the midway position between Europeans and East Asians.

This does not mean that I believe that before 5,000 BC the gene flow between East Asia and Western Eurasia was zero. Rather, I think there are lots of data which imply that it was simply very low (the East Asian admixture among Tatars in Russia, and the West Eurasian admixture among Mongols, both show evidence of being relatively recent, due to the rise of horsemanship). This is in contrast to the more genuine cline and isolation-by-distance you see from Europe down to the Middle East, and to a lesser extent South Asia. Actually, until recently I would have said into South Asia without qualification, but I am now convinced that South Asia itself has been the scene of an admixture event of huge scope within the last 10,000 years.

Much of the discussion that Jason Antrosio alludes to discusses the problems which have emerged from hypothesis based admixture inference programs, such as Structure, frappe, and Admixture. The main issue is that many people read them naively. This includes people in the academic community. But this does not mean that no one understands the problem. I’ve talked to evolutionary genomicists who have complained about the misinterpretations, and I am quite aware of the artifacts which can flow out of the software. Anyone who has used Admixture knows very well the problems. For example, South Asians often emerge as a distinct cluster, but the research above indicates that they are a stabilized hybrid! This is why I told some of Antrosio’s commenters to be careful about hitching their wagon to isolation-by-distance and clinal variation; there is some evidence that many of the world’s populations extant today are the product of relatively recent hybridizations between previous rather distinct groups. There’s no need to invoke Platonic original races. Rather, it may simply be that in the random lottery of cultural adoption some groups invented agriculture, and replaced many populations which exhibited a clinal variation.

And that’s the key: racial typologies are coarse reflections of genuine history. In other words, race is a reflection and reification of genuine lower level dynamics, it is not the prior phenomenon. This sidesteps many of the technical complaints which arise in the papers Antrosio linked to. I can quibble with them well enough though. For example, figure 2 in the Genome Research paper relies upon a rather shitty (in relative terms) genetic relatedness statistic, IBS, in my opinion. Don’t take my word for it, play around with data sets in Plink and you’ll see what I mean. It tends to be history-blind. My parents, who are South Asian, but with a non-trivial East Asian component, are often clustered with a host of other South Asians who also have non-trivial East Asian components. This is a real result, but it ignores the history that all that is common across these individuals is a particular admixture pair. It’s not a “real” cluster, reflecting real shared history.

A more interesting concern is the fact that in most trees non-Africans tend to be on their own branch, while Sub-Saharan Africans tend diversify into distinct basal branches. The question ensues: are Sub-Saharan Africans several distinct races? Using evolutionary history as a measure I would say yes! This is definitely one area where social expectations have led us astray. It turns out that it may be that the Bushmen/non-Bushmen separation is only 1/3 as long ago in the past as the Neanderthal/modern human separation. In fact, the Bushmen may predate, and not be part of, the “Out of Africa” event. Along with the Pygmies and Hadza there seems to be a very ancient differentiation between the agriculturalist and hunter-gatherers in the African continent.

For me these details of history are fascinating. But going back to normative concerns: is there a worry that Bushmen will be dehumanized if it is understood that they are not part of the modern human expansion event circa ~80,000 years before the present? Unfortunately, I don’t think that science matters much in this case. The Bushmen have been dehumanized for hundreds of years. The Pygmy of Central Africa have also been dehumanized. All without science. An understanding of our evolutionary history is informative, but I doubt it is the prime motor for the great injustices of history. The 19th century race science which modern biologists and anthropologists revile (to a great extent, rightly) did not give rise to the race system of the West. Look at the history, and you see that its genesis predates Darwin by decades. Science may have been a supporting argument, but this was thesis looking for talking points.

The Bushmen are human. The Bonobos are not. Why? I don’t think it has been definitively proven that modern humans and Bonobos are not inter-fertile. Granted, the separation between the Bonobos and humans are about two orders of magnitude greater than Bushmen and other humans, but there is some evidence that Bushmen have admixture from archaic lineages diverged nearly 1 million years into the past, pushing elements below a magnitude! Where do you draw the line? Species are a typological concept, but usually as a pure categorical typology the class is useless. Rather, it’s a tool, a framework. What you do with a tool, well, that’s a different thing altogether….

February 12, 2012

The social and biological construction of race

Filed under: Anthroplogy,Hispanics,Latinos,Population genetics,race — Razib Khan @ 2:45 pm

Many of our categories are human constructions which map upon patterns in nature which we perceive rather darkly. The joints about which nature turns are as they are, our own names and representations are a different thing altogether. This does not mean that our categories have no utility, but we should be careful of confusing empirical distributions, our own models of those distributions, and reality as it is stripped of human interpretative artifice.

I have argued extensively on this weblog that:

1) Generating a phylogeny of human populations and individuals within those populations is trivial. You don’t need many markers, depending on the grain of your phylogeny (e.g., to differentiate West Africans vs. Northern Europeans you actually can use one marker!).

2) These phylogenies reflect evolutionary history, and the trait differences are not just superficial (i.e., “skin deep”).

The former proposition I believe is well established. A group such as “black American” has a clear distribution of ancestries in a population genetic sense. The latter proposition is more controversial and subject to contention. My own assumption is that we will know the truth of the matter within the generation.

A black American

But that is the biological construction of race. Subject to fudge and fuzziness, but mapping upon a genuine reality. What about the social construction? Due to its flexibility this is a much more difficult issue to characterize in a succinct manner. Consider the cultural conditionals which render G. K. Butterfield “black” and Luis Guzman “Hispanic.” Both individuals are products of an admixture between people of mixed African and European ancestry (and likely some Amerindian in Guzman’s case). It turns out that the genes have segregated out such that Butterfield reflects more his European ancestry in traits. Guzman’s phenotype is more mixed. The perception of these two individuals is weighted by two different strains in modern American racial ideology. First, that of hypodescent, where one drop of black blood means that an individual is black, without equivocation. Halle Berry appealed to this framework to argue why her daughter, who is less than 1/4 African in ancestry (Berry’s African American father almost certainly had some European ancestry) was black. No matter that hypodescent’s origins were to buttress white racial supremacy and purity. Today black Americans espouse for purposes of community solidarity (the black American community as we know it is a partly a product of hypodescent which forced mixed-race blacks into the African American community).

Not a black American

The second issue, which has crystallized in our time, but has roots back decades, is the peculiar position of “Hispanics/Latinos” in the American racial system. As A. D. Powell has observed Hispanics seem to be able to evade the one drop rule, unless their African features are extremely dominant (e.g., pre-skin whitening Sammy Sosa). I’ve looked at the genotypes of enough Latin Americans to assume that some level of African ancestry (e.g., ~5%) is present in the vast majority of those who are not the children of recent European immigrants or from indigenous communities. For example, Mexico’s large slave population seems to have been totally absorbed, to the point where their past existence has been nearly forgotten. Mexicans of mestizo or white identity routinely have African ancestry, they just don’t know it, nor is it part of their racial identity. And it isn’t just Latinos. People of Middle Eastern ancestry, in particular Arabs, often have some African ancestry. But they are not classified as black (unlike Hispanics/Latinos they don’t have their own ethnic category, but are put into the “white” box, irrespective of their race, from Afro-Arab to Syrian).

This broader coexistence of frameworks persists on the implicit level. We don’t usually explicitly flesh out these details. Rather, we take these social constructions as givens. The major problem is when the problems and artificialities of these social constructions begin to bleed over into attempts to understand patterns of biological variation. Because of America’s fixation on the black-white dichotomy rooted in skin color people routinely offer up the fact that the human phylogeny is not well correlated with pigmentation as a refutation of the concept of race. What biology is doing is refuting a peculiar social construction of race. It is not negating the reality of human population substructure. Sociology and culture anthropology are empires of imagination to a much greater extent than human biology.

I’m thinking of this because with the birth of my daughter I confronted the bleeding over of the social into the biological. For medical purposes her race had to be assessed. One side of her ancestry was not problematic; white European. But I had to argue for why her other half should not be listed as “Asian.” For sociological purposes I have no great issue with the term Asian American which is inclusive of South and East Asians (I am not denying that this a recent political identity, I am saying that I do not personally find it objectionable and routinely enter my race as “Asian American” into public forms). But for biological purposes this is an incoherent and misleading classification. I know when my sister was born my parents put her race as “Asian,” which even at the time I felt was totally without purpose as far as biological taxonomy went. At the end of it all my daughter had “South Asian” entered in by hand. Better that her information be discarded than aggregated into a data set in a misleading fashion.

Obviously disentangling the social and biological is not necessarily impossible. Rather, it takes a little care and explicitness, as it is so easy to move between the two domains so easily as to elide their differences. And to some extent they do inform each other. Personal genomics is adding a new twist, but the general problem is as old as human systematics. The only cure is care.

Image credit: Wikipedia

January 16, 2012

Mendelism is not magic

Filed under: Genetics,Genomics,Human Genetics,Human Genomics,Pigmentation,race — Razib Khan @ 1:23 am

Michelle points me to this article in The Lost Angeles Times, The Colors of the Family:

I was holding my 1-year-old, ambling about downtown with some friends. White friends. She must have thought my boy belonged to one of them.

There’s a simple explanation: I’m black but my son, Ashe, is white. At least he looks it.

But things are more complicated than that.

I’m actually half black and half white. It should come as no surprise, though, that even as sophisticated as we’ve become about people of mixed parentage, I’m pigeonholed as black. If someone asks and I don’t have time to go deeper, that’s what I call myself.

Ashe is mixed too. His mother, my wife, Vanashree, is half white and half South Asian, with roots in India. She has olive skin, and Ashe is slightly lighter than she is.

This surprised us. When Ashe was born, one of the first things I said to Vanashree was, “Honey, he’s so light!” We chuckled, poking fun at our assumptions.*

Let’s get the sociological aspect out of the way. Is this really that surprising? Folk-biology has always had the concept of a “throwback,” which really distills the reality of Mendelian inheritance (as opposed to simple blending processes). In societies such as Brazil or India where there is a fair amount of segregation of polymorphisms which control skin color it isn’t that unheard of for a child to be darker or lighter in tone than both parents. And more frankly, this is not unknown within the African American community, where there is a range of skin tone due to ~20% European admixture. I suspect many African American would have these “assumptions,” because of an intuitive understanding of the unpredictable nature of the inheritance of this trait.

Second, the author of the piece is half black and half white in social terms, but there is no chance he is 50 percent African in ancestry. Barack H. Obama is 50 percent African in ancestry, but African Americans almost always have some admixture. I’ve analyzed ~150 African Americans in terms of their ancestry, and they always have some European ancestry. In fact the few Africans in my data set jump out because they lack this component. In other words, the author’s child is somewhat more than 50 percent European in ancestry.

Finally, what’s the science behind this? This isn’t that  hard to actually understand, because the genetic architecture of pigmentation has been well elucidated. Only a few genes control most of the variation across populations (the difference we see between Africans and Europeans, South Asians and East Asians). Because we know the parents’ ancestry we can make a few educated guess.es The largest effect size upon of a gene pigmentation in a given individual is probably from SLC24A5. The father is likely  a heterozygote on this at the SNP in question, with a “light” European copy, and a “dark” African one. The mother is most likely, though not inevitably, a homozygote; the frequency of the “light” copy is well north of 50 percent in South Asians (I’m a homozygote, as are both my parents). So the child has a 50 percent chance of being a heterozygote or a “light” homozygote. That’s some of the answer right there. Because the child does not have blue eyes we know that they are unlikely to be homozygote for the combination of markers which is correlated with blue eyes (probably due to a regulator element on the HERC2 locus). This is also associated with lighter complexion and hair color. But there is another locus which I think would be especially important: SLC45A2. There is a “light” variant here which is highly localized to Europeans. Its frequency is 95 percent in Northern Europe, and 15 percent in Northern India (85 percent in Northern Italy, 65 percent in Turkey, etc.). It is not found in East Asia or Africa, except in cases of clear admixture with Europeans. Europeans who are homozygote for the “dark” variant tend to be olive skinned (this genotype is relatively rare, though not unheard of in Southern Europe as per the frequencies above). Both the parents in this case would almost certainly be heterozygotes. This means that their son had a 25 percent chance of exhibiting the Northern European genotype. That is a straightforward explanation for why he might be lighter than either parent. Of course there are a few other genes of some importance, but I suspect that SLC45A2 is where most of the work is done in this case because of the backgrounds of the parents (i.e., I’m pretty sure they’re heterozygotes).

I understand that the point of the article was not the genomics of pigmentation. But to talk about social matters it sometimes pays to get the science nailed down. Like it or not this is a time in the United States where people of mixed ancestry are going to be more common. I rarely get the “Where are you from?” question anymore (because I’m not black or white), but I wonder if the “What are you?” (asked of mixed-race individuals) is going to persist a little longer.

* I think lurking within the subtext of the article is the salience of African ancestry, and the idea that it is particularly potent. The author’s wife’s background is mentioned almost in passing, before moving back to the main attraction of the child of an African American no longer appearing visibly African American. Many of the ideas of white nationalist thinkers such as Madison Grant may no longer be in vogue, but their idea that African ancestry was particular powerful in swamping out all other ancestry remains an unspoken assumption in American society.

October 13, 2011

The future is not impossible

Filed under: Genetics,race — Razib Khan @ 10:32 am

Alexander Dumas, of mixed race

One of the reasons I post regularly on the genetics of mixed-race people and their physical appearance is that I don’t think the media does a good job. There’s a “freak show” element which titillates but does not illuminate. This in a period in the United States where the absolute number of people of mixed origin is increasing rapidly due to intermarriage. In fact for years I’ve gotten inquiries from the parents of mixed-race children about the scientific details of the genetics, because they are regularly questioned in depth as to how the children came to look how they look (the emails are always from women, more on that later). A relatively well written article in The New York Times illustrates some of the issues, insofar as the focus is totally on social context and dynamics, with not even a small nod to the science. The story is about a mixed-race woman (mother white and father black) who is married to a white man, whose children don’t “look black.” Specifically, her two daughters are very light-skinned, and the younger one is boldly blonde.

Here’s the jump off point of the piece:

“How come she’s so white and you’re so dark?”

The question tore through Heather Greenwood as she was about to check out at a store here one afternoon this summer. Her brown hands were pushing the shopping cart that held her babbling toddler, Noelle, all platinum curls, fair skin and ice-blue eyes.

The woman behind Mrs. Greenwood, who was white, asked once she realized, by the way they were talking, that they were mother and child. “It’s just not possible,” she charged indignantly. “You’re so…dark!”

It was not the first time someone had demanded an explanation from Mrs. Greenwood about her biological daughter, but it was among the more aggressive….

Of course it’s possible. The science behind this is trivially plain. The biological mother has alleles which code phenotypes distinctive of Europeans and Africans. Because her father is African American she is even likely to have more European ancestry than African ancestry (median African American is ~80% African and ~20% European). Genes which control variation in skin pigmentation at the scale of racial differences are distributed across half a dozen loci, but with blue vs. brown eye color there’s really only one locus which explains most (though not all) of the variation. That probably explains how both the daughters have blue eyes. The mother is probably a heteozygote, and the father is a homozygote. That means that any one of their children has a ~50% chance of having blue eyes and a ~50% chance of having brown eyes. So the chance of both daughters turning out to have blue eyes is ~25%. But obviously the science isn’t the meat of the piece. I just wish they’d given a quick explicit nod to it so that people would know why the outcome is as it is. It’s not rocket science.

In terms of the experiences of this family, there are a few issues that come to mind. First, it would be nice to clarify the ages of the individuals who ask her rude questions. My personal experience is that people who compliment me on my ability to speak English well have invariably been born before 1970. No matter one’s politics or current life situation formative experiences matter. These are people who grew up in the United States before there were large numbers of children in their classrooms who “didn’t look American” (i.e., they weren’t black or white). In contrast, people who are younger had some of these kids in their classes growing up, so the fact that we don’t have accents and can speak English fluently isn’t an amazing feat which requires a compliment.

Second, I wonder as to the dynamics in this specific case, a mixed-race women who is coded as black in the United States. It doesn’t seem implausible that the experiences of Asian American and Latino women who have very light-skinned children would be somewhat different, because there’s a lot less fraught history there. The power of hypodescent still looms large in black-white relations, in a way it necessarily does not with other groups.

Finally, there’s the issue of mothers versus fathers. These stories always seem to focus on mothers who are assumed to be nannies of their own children. This is a very specific fear which I’ve read some black women have when it comes to interracial dating and the “think of the children” angle. But what about the fathers? It seems that paternity is something which is going to be much more plausible to challenge. In the few cases where black American families have adopted white children I’ve read that they have to be very cautious in public lest people get the wrong impression, and this is a particular problem with black males (like some gay couples keeping adoption papers on one’s person can be handy in these instances).

But that’s all sociology. There’s a lot of moving parts. But the genetics of this isn’t too complex. Quite possible indeed.

October 5, 2011

Mixed-race people are mildly complicated

I was pointed today to a piece in the BBC titled What makes a mixed race twin white or black?. The British media seems to revisit this topic repeatedly. There are perhaps three reasons I can offer for this. First, it tends toward sensationalism. Even though the BBC is relatively staid, when it comes to science it converges upon the tabloids. Second, because the number of non-whites in Britain is relatively small, there is a higher proportion of intermarriages between minorities and the white majority (from the perspective of minorities). This is especially true of people of Afro-Caribbean ancestry. So of the proportion of minorities a larger fraction are recently mixed in Britain than in the USA. Finally, the United States has a more complex attitude toward race relations than the United Kingdom, because the former has traditionally had a large non-white minority while the latter has only had so since the years after World War II. I suspect that “black-white twins” stories would seem in bad taste on this side of the pond, and bring up certain memories best forgotten.

Now, there are fallacies, confusions, and misleading shadings, in the BBC piece. I’ll hit those first before reviewing what’s going on here when fraternal twins exhibit totally different complexions.

It starts out somewhat ludicrously: “Her son Leo has black skin and her daughter Hope, has white skin.” This is false in a precise sense. Leo clearly has medium to light brown skin (there are photos in the piece). What’s going on here is that Leo has some African ancestry, and because of the rule of hypodescent all people of African ancestry with a shade of brown skin, from nearly black to light brown are termed “black skinned.” This is not a trivial semantic elision. If Leo truly had black skin, very dark brown, than there’d be a lot of explaining to do, because the genetics would be somewhat mystifying. More on that later.


She was adopted when she was four years old, and her birth mother is Afro-Caribbean and her British birth father was white. Her DNA tests revealed that, genetically, she was exactly 50% African and 50% European.

This is very unusual, and the results suggested that Shirley’s mother had pure African roots, and that her ancestors must have moved from Africa to the Caribbean quite recently.

Not necessarily. Mixed-race people, especially those with recent admixture, don’t have their different ancestral components distributed equally across their genome. It may be that in the process of sampling chromosomes from this individual’s Afro-Caribbean mother she received almost none of the European quantum, perhaps localized to a few chromosomal segments. This “noise” in the process explains why I seem to carry an elevated proportion of East Asian ancestry in relation to both of my parents. I simply received genetic copies sampled from the more “East Asian” regions of my parents’ genomes.


“Our skin colour is determined by a number of gene variants – at least 20 variants, I would say, probably quite a few more than that,” says Dr Wilson.

This is complicated, but I’d say that the good doctor is misleading the audience. Skin color seems to be a quantitative trait where you can explain the vast majority of between population variation with only a few genes, at most six. When it comes to European-African difference variants at two loci, SLC24A5 and KITLG can account for well over half of the difference. It is true that there are many, many, genes that effect skin color, but there is a definite distribution where the vast majority of genes tweak the trait only on the margins. In other words, there may be 20 variants (there are more), but for good predictive power at the inter-population level you’re good to go with 4 or 5.

I specify inter-population level, because within populations the gene set which can allow you to predict variation may be slightly different, and you have to take into account sex differences. For hormonal reasons males seem somewhat darker than females in human populations. Additionally, people also are palest in their youth, and become darker as they age. Finally, some of the genes which explain differences between populations are invariant within a population. Therefore the genes which are of lower effect size move up the stack. So when it comes to European-African variation, the largest effect gene, SLC24A5, won’t explain anything within these two populations. That’s because it is fixed for alternative variants (the light vs. dark conferring variants). So the second effect size may move up to first effect size when you evaluate on a smaller grain (but if the second effect size is nearly fixed, then it might drop far down as well).

Now let’s move on to the common idea that darkness dominates over lightness:

As in a painter’s palette, in the skin the presence of pigment dominates the absence of pigment, so the fact that Hope is white is very unusual.

This is hypodescent popping up again. Though in the West we live in an anti-racist age, at least notionally, it is interesting how concepts and models from a white supremacist era remain operative, at least implicitly. The idea that whites are recessive to non-whites makes totally sense if you code anyone with visible non-white ancestry as non-white. Even if they are genetically more white than not. The rationale for this model was the idea that there is a reversion to the more “primitive” type. So a cross between a black and a white produced a black, and a cross between a Nordic and a Mediterranean produced a Mediterranean. Inferiority taints the purity of superiority.

Less ideologically if you classify skin complexion into white and non-white in a dichotomous fashion then you logically consign the non-white trait to dominance. For example, if nearly, but not quite, white skin is “dark,” then you make it very difficult for someone with a substantial number of pigment conferring alleles to produce a child with very light skin.

Finally, now that we have elucidated the genetic architecture of pigmentation to a great extent we can make assessments of dominance and recessiveness on a locus by locus manner. If you plot skin complexion darkness as a function of reflectance you can turn it from a dichotomous or discrete trait to a continuous one. So individuals can have a “melanin index,” an integer value equivalent to their position on a scale of lightness and darkness. Converse to expectations above it turns out that on the two largest effect genes explaining difference between Africans and Europeans the light alleles are more dominant than the dark alleles! In other words, if the two alleles had an equal effect you’d expect a value between the two in their homozygote state. As it is, the values tend toward higher reflectance (light) than dark. I would caution that terms like “dominant” and “recessive” can be highly subjective and dependent on how you code the trait, the nature of the population you sample from in a polygenic character, or even scale the of values. So in this case you notice that switching from a dichotomous code of white vs. non-white to a continuous value corresponding to reflectance flips the model from the light trait being recessive to the dark, to the dark being recessive to the light (albeit, only mildly).

Because pigmentation is controlled by only a few genes the state at these loci are poor proxies for total genome content. In plainer language mixed-race siblings won’t deviate too much in their ancestral quantum, but they can deviate a great deal in their physical appearance. In fact, because of the poor correlation the slightly “blacker” twin in total ancestry may actually look more like a white person, and vice versa.

Now let’s go back to first principles. We’ll make some simplifying assumptions to illustrate what’s going on easily. Take 6 genes which control skin color. Assume equal effect. Each gene comes in two variants. Light and dark. Two copies of light result in a value of 0, while two copies of dark result in 2. A copy of each results in 1. In other words, the alleles are additive across a locus. Also assume that the genes are independent. They’re not linked. So the value at each gene is independent of the other genes. Finally, assume that the genes’ implied values summed together result in a total pigmentation phenotype outcome. So they’re additive across loci.

To make even simpler let’s assume that the parents are F1 African-European hybrids. That means that one of their parents’ was European and the other African. So both share the same ancestry of recent vintage. As it happens Africans and Europeans are very different on pigmentation genes, so we can assume that these parents carry one light copy and one dark copy across the six genes. This means you’d expect them to be brown.

Since they are brown, wouldn’t their children be brown? No. Not necessarily. As per Mendel’s Laws each contributes contributes one gene copy at each locus. So for the 6 loci above each parent contributes one pigmentation gene. What does that mean concretely? I Already simplified things to produce an elegant outcome: the F2 offspring could be all light, all dark, or one copy of both, like their parents, at any given gene. To illustrate what I’m talking about, SLC24A5 is disjoint in frequency across Africans and Europeans. All Europeans have one variant, and all Africans have another. So the offspring of a marriage between an African and a European will be heterozygote on that locus. If they marry another person of similar background, homozygote light and dark genotypes will resegregate out at fractions of 25% each, with half the outcome being heterozygote as in the parental condition. In other words, there are a 25% probability of a F2 child of F1 hybrids being “white” at this locus. There are 6 loci. Assuming independent probabilities, you multiply out 0.256, and get 1 out of ~4,000 that the child will be white like their white grandparents.

I ran this as a binomial 10,000 times, and here’s the distribution I came up with:

The white and black offspring don’t show up because the number of outcomes is so rare in this model, but as you can see the median outcome is brown, like the parents. But the tails are significant. In other words, don’t be surprised if there’s a lot of variation among the siblings. But why should you be? If you know of people from populations where pigmentation alleles are segregating in polymorphic frequencies, such as Latin Americans and South Asians, you are aware that different siblings can look strikingly different when it comes to complexion. Though I guess that’s a new insight for the British….

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