26 May 2017
Genetic Admixture and Gender Asymmetry
(I couldn't help myself and did let some friends know that I was starting a blog, or two actually. Younger, former colleagues immediately asked "What is it about?" What a notion, I thought, a topical blog! I just answered, evasively, "About nothing in general, and everything in particular." My newer, but older weekly friends, didn't ask. They know me: topicality is not my thing.)
Now that genomic studies are affordable, people are learning about their genetic ancestry, which by itself is quite fascinating and may even be useful, as they awaken the curiosity for the past from a personal perspective ("I didn't know that I was..."), and they also help to pinpoint genetic traits and related health issues. (They also raise very valid privacy concerns, but that's a different discussion.)
The past, of course, is disentangled with the help of Clio, if that's the name of the muse of history. If John Doe is 28 percent Celtic Scottish and 40 percent Scandinavian and so and so, including 2.8 percent Neanderthal, something happened in the past to explain the makeup of John's DNA. It's near impossible to know if centuries-old admixtures happened because of love, war, casual encounters, rape, seduction...
Notwithstanding, it's possible to submit common scenarios that may explain many older processes of genetic admixture. War and conquest, especially before the late 19th century, resulted very often in admixture by force, with the expected gender asymmetry: the defeated soldiers, male with rare exceptions, were killed, enslaved or pushed away, with the winners having children with the women of the losing side. If they belonged to different genotypic groups ("races"—whatever the meaning of the term—being only one of the difference markers), admixture results. The process of admixture usually continues for many generations, as societies tend to ascribe more prestige and desirability to belonging in the winners group. Phenotype, language, customs, occupation, and so on, become a mark of prestige or contempt. Upward social mobility is helped when some of the desired traits are acquired or learned.
The reader, at this moment, may be thinking about admixture of "black" with "white," or European with Native Americans; that’s understandable, given the fact that those are still historically fresh phenomena. Also, the reader may be thinking of the word "miscegenation," which was part of my accented English until very recently, when I learned that its connotations had been and could still be viciously negative among many people in the United States (said word apparently coined in a pro-admixture New York pamphlet around the early 1860s—Civil War's time). But genetic admixture is of course quite older than that recent word and the 5-plus centuries-old America-Europe exchange.
Examples abound in every period of history and in every continent. In most of Europe, starting with the very arrival of Homo sapiens sapiens, there was displacement with genetic admixture (hence the Neanderthal traces in many people with European ancestry today); the Celts displaced and mixed with the Iberians; the Romans conquered and left their Y-chromosomes all over Europe and beyond; the Germanic Goths did the same (turning the pejorative "Gothic" into a prestigious qualifier), only to be displaced by the Normans, initially thought of as fierce and brute conquerors, later becoming a desirable and superior "race" (in the words of the nobility from them descended, of course).
Gender asymmetry in places like Europe did clearly exist, but it may be difficult to read its history in the current genomes of Europeans, given the back and forth conquests that happened there in the six millennia of recorded history. The vanquished (providing female mitochondrial DNA in the resulting admixture) of one time may later become the victors (imposing their Y-chromosome). That may also be the case in many other Eurasian populations, given the many displacements of people from East to West and vice versa. Notwithstanding, there are some notorious genetic markers that have persisted until now. One (in)famous case is that of a man that lived around 750 years ago, whose Y-chromosome is still present in 16 million men across Asia, from sea (the Pacific) to shining (Caspian) sea. The originator of that feat was probably Genghis Khan himself, who usually slaughtered much of the male population of the places he conquered.
It's not necessary for war to continue for the gender asymmetry to persist and further develop. In many cases, prestige based on wealth or power may reinforce this type of bias. In some of West Africa's mixed populations, for instance, a 2013 study shows a clear prevalence of Y-chromosome from farmers over hunter-gatherers, compared with the mtDNA proportions of both groups.
That gender asymmetry is quite more evident in the most recent cases of genetic admixture, as is the case in the Americas. Due to multiple factors, particularly during the first two or three centuries after contact, including openly promoted genocide, war, rape and sexual slavery, forced labor, and prestige ("improving the Indian race"), the natives of the Western hemisphere had a very biased reproductive success: the closer the native males were to the centers of colonial domination, the fewer opportunities they had to procreate and thus pass their Y-chromosomes to their children, while the females usually bore children with the male occupiers, especially early in the process when European women were almost totally absent in the colonial settlements.
Genetic studies have provided very telling results that have surprised Tyrians and Trojans alike. A case in point is the island of Cuba: most Cubans see themselves as descended from Europeans and Africans, and a few Asians (Chinese, especially). I have not seen a single reference of Cubans identifying themselves as indigenous American in any proportion. Genes, of course, do not care much about their owners' personal opinions, and they show the real deal: a sample of over 1,000 Cubans showed that 39 percent had African mtDNA, 26 percent Eurasian (mostly European, for sure) mtDNA, and a very unexpected 35 percent carried indigenous mtDNA! On the male side, the same study’s sample of nearly 400 males with genotyped Y-chromosomes presented a very different proportion: nearly 82 percent had male European ancestors, 18 percent had African male ancestors, and only two individuals (0.5 percent) had direct American indigenous paternal ancestry (Marcheco-Teruel et al., 2014). Putting it in simpler terms, 74 percent of Cubans in that sample have direct maternal ancestry from African or Native American women, and at the same time 82 percent of them have direct paternal ancestry from Eurasia. It's very difficult to find that level of gender asymmetry anywhere else.
In general, Latin American genetic studies show that the proportion of indigenous mtDNA is quite more prevalent of what could be expected. A couple of examples: A study in Puerto Rico, also considered a population of mostly European and African descent, shows that over 60 percent of a sample had indigenous (Taíno) mtDNA (Martínez Cruzado et al., 2005). In Argentina, considered by many a mostly European country, over 50 percent of people in a study had indigenous mtDNA (Bobillo et al., 2009).
But what about the countries where the population is very visibly indigenous or mestizo (as some of us call ourselves)? Mexico, most of Central America and the Andean region, dominated by large and powerful polities before the European invasion, suffered heavy demographic losses due mostly to disease. But indigenous and mestizo people rebounded and constitute now the majority in most of those countries, even though they have been and still tend to be governed my a minority of mostly European descent, which still commands power, wealth and prestige, even though not at the same level of, say, a century ago. That domination imprinted in many Latin Americans the desire to "improve the race," usually marrying daughters with "whiter" men. Thus, it is to be expected that a strong asymmetry will be present, favoring indigenous mtDNA and Eurasian Y-chromosomes, with the actual phenotypical features not necessarily reflecting the maternal and paternal lineages.
As somebody born and raised in a country where admixture has been the norm for 500 years, my expectation was that I would conform to the asymmetrical lineages, even though my mother's family was apparently more European, and my father's more indigenous. That was confirmed by the results of my DNA analysis: I carry American indigenous mitochondrial DNA and European Y-chromosome. The rest of my genome, after centuries of admixtures in both sides of the family, happens to be 50 percent indigenous American and 50 percent European: a perfect mestizo, by pure chance.