Elite kin groups down the generations

The Pith: The purported sons of great men often are really the sons of great men. Another case of “Conan was right”.

Dienekes points me to a neat new paper, Present Y chromosomes reveal the ancestry of Emperor CAO Cao of 1800 years ago, which attempts to validate the claims to descent from a particular ancestor by a set of Chinese clans. The Chinese clan system is based on direct paternal descent by and large (and there has been a history of aversion to adoption from outside the kin group), and so aligns perfectly with the phylogeny of Y chromosomes, which are passed from father to son to son to son. That’s the ideal. What’s the reality? People are adopted. Or, some sons of a purported father are actually not the biological sons of that father. And finally, there are cases where individuals may fabricate ancestry and interject themselves into a lineage group through deception.

The individual in this case flourished 72 generations ago. Additionally, there is some controversy as to the relationship of this individual to others of their lineage. Here is the important section from the paper:

Here, we typed 100 Y chromosome single-nucleotide polymorphisms (Supplementary Table 1) as listed in the latest Y-chromosome phylogenetic tree…on 280 individuals of 79 Cao clans or clan clusters from different locations throughout the China…and 446 individuals of different clans with other surnames. A clan cluster may consist of several simplex clans if they carry different Y-chromosome haplotypes. Thus, we studied overall 111 simplex clans of CAO (Supplementary Table 2). According to their stemma records, 15 of the CAO clans claimed to be descendants of Emperor CAO. These 15 clans distributed in different provinces and never knew the existence of each other. Their Y chromosomes comprise six haplotypes…Only one of these six haplotypes can be Emperor CAO’s type. The other haplotypes found in the claimed clans might be introduced by other sources such as adoption, acceding to mother’s surname, nonpaternities, and so on…Here we need to recognize the most probable Emperor CAO’s haplotype by examining the haplotype distribution among the clan groups.

The strategy here is simple. You have a set of individuals who claim descent from a putative ancestor down the male line. If the social ideal matched the biological reality these individuals would share the same Y chromosomal lineage (diverged by mutations in direct proportion to time since the last common ancestor). Reality never matches the social ideal. But if that reality has any basis in fact you will see an over-representation of one lineage among the diverse set of lineages. That’s because the interjected lineages should be a relatively random sample from the population, slowly replacing the ancestral lineage. The authors found that pattern. In addition, they noticed that there was a different lineage prominent among those who were of the broader Cao clan, but not descended from Cao Cao. Therefore, they concluded from these results that Cao Cao himself was not a genuine member of the Cao clan, though many of those claiming descent from him are genuine. He was a case of fabrication of ancestry, witting or unwitting. Here’s a table illustrating the results for the marker hypothetically associated with Cao Cao:

I assume that with a deeper sequencing of the Y chromosome this sort of analysis will get much better. From a population genetic perspective this isn’t interesting really. But from a social historical perspective it is. There are many groups around the world which claim descent from a male founder. The distribution of genuine vs fictive groups could give us a sense of the persistence of prestige across the generations, and the ability of a given kin group to defend its exclusive prestige against newcomers who might attempt to erode or co-copt it. It is notable that 6 out of 15 clans who claim descent from this individual 72 generations in the past are actually descendants! If you assume that 40% of the hypothetical descendants are actual descendants then you have a fidelity of ~99% generation. That’s pretty impressive, but is about typical for high status males in terms of rates of cuckoldry today.

Hopefully we’ll be checking for the Sayyids in the near future. I suspect that there’s been a lot more obscuring of the genetic signal in this case for various reasons. As more and more “elite lineages” get typed we can modify our assessments of the likelihoods of prior results, such as that of Genghis Khan.

Citation: Journal of Human Genetics, doi: 10.1038/jhg.2011.147

Image credit: Wikipedia