8% of Asian men are descendants of Genghis Khan

President Obama has been reproached for not being African American, despite having 50% of his DNA inherited from an African father and the other 50% inherited from a mother with European roots. In fact, there are African Americans who have a lower percentage of African DNA in their genome than he does, but socioculturally it is considered that only the descendants of African slaves who were kidnapped and transported to America as forced laborers are African American.

The vast majority of African American people in the United States have a high percentage of DNA of European origin, because many enslaved women were raped or became pregnant by their masters. The different European and African genetic contribution can be demonstrated through genetic analysis using forensic DNA techniques.

Forensic genetics allows for the identification of biological samples and the resolution of paternity/maternity cases, but it also enables historical studies and human migration research. We know that the mitochondrial DNA we possess comes only from the biological mother, who in turn has the same DNA as the maternal grandmother, and so on. Therefore, mitochondrial DNA explains matrilineal lineages. On the other hand, boys inherit the Y chromosome from their biological father, who in turn received it from his father, and so on, from their male ancestors. Thus, Y chromosome DNA explains patrilineal lineages. In many current human populations, we can identify the ancestral genetic footprints of migrations, wars, and colonizations forgotten in the mists of time.

The imprint of the violations

A few years ago, a genetic study of the genetic remains from a chultún (a type of subterranean ceremonial cenote) in Chichén Itzá, where there were a hundred skeletons of children between 3 and 4 years old of male biological sex sacrificed in religious rituals in the pre-Columbian era, demonstrated that their mitochondrial DNA and most of the other chromosomes (around 92%) were very similar to those of the current inhabitants of the Yucatán Peninsula (Mexico), except for the Y chromosome, since around 50% of current men in the Yucatán have a European Y chromosome, which demonstrates the asymmetry of the initial interbreeding during the Spanish colonization. The colonizers, mostly males, replaced almost half of the male parental contribution in the interbreeding with indigenous women.

In other parts of the world, genetic contributions due to wars can be found, such as in Lebanon, Syria, and Jordan, where one can find Y chromosomes that originate from Brittany and Normandy, which demonstrates the genetic footprint of the crusaders during the various centuries of the Crusades. Or with a more global view, 8% of all Y chromosomes in Asia have the same origin, as they are men descended from Genghis Khan, a warrior who not only killed many other men in the conquered lands but also boasted of having more than a thousand children, probably through the rape of many women in the occupied territories. His children and descendants, also warriors, did the same, and this explains the worldwide spread of his Y chromosome today.

A controversial case for centuries

The third president of the United States, Thomas Jefferson, was an advocate for the abolition of slavery, but he had more than 600 slaves throughout his life, who were sold when he died. It is estimated that, in his lifetime, he freed fewer than a dozen, among them the children of a much younger slave of his, Sally Hemings. Hemings was the daughter of the concubine, also a slave, of a wealthy man of Caucasian origin, Jefferson's father-in-law; therefore, Hemings was the half-sister of Jefferson's wife on their father's side. It all stayed in the family.

For two centuries there was speculation about whether Hemings' children were biological children of Jefferson. The controversy was served: an anti-slavery advocate having children with his slave, without acknowledging them as his own children. Historical studies went in opposite and conflicting directions: there were historians who argued that they were children of Jefferson and others who denied it. In 1998, a forensic genetics study of the Y chromosome of Jefferson's paternal family and male descendants of two of Hemings' sons demonstrated that at least in the case of the last son the Y chromosome was the same and that, therefore, very likely Jefferson had children with Sally Hemings. Detractors of this theory argued that the Y chromosome does not prove direct paternity, as any other man in the Jefferson family could have been the father.

Well, a very recent study, using forensic genetics techniques of massive sequencing of the current descendants of Sally Hemings, has compared DNA fragments from many chromosomes to infer their genetic proximity to the DNA of Thomas Jefferson, obtained from a lock of hair. The comparative study also included current descendants of Jefferson with his wife, Martha Wayles, to analyze the amount of DNA shared with the descendants from one side and the other, after two centuries. The conclusion, probably expected but now genetically demonstrated, is that Thomas Jefferson really did have children with his slave, since the current descendants of Hemings share with him an amount of DNA very similar to that of the descendants of his recognized children. History can take many turns, but forensic genetics can resolve enigmatic and controversial cases like this one.