A few days ago I suggested that Dr. Daniel MacArthur might have South Asian ancestry. Now, when confronted with surprise the best option is to stick with your prior assumption, unless that surprise is powerful enough for you to “update” your model. After a few days of further analysis I will update: I do think Dan MacArthur has South Asian ancestry. Dienekes dug further, and noticed that there are hallmarks of “Ancestral South Indian” ancestry along the first 2/3 or so of chromosome 10. Now, you do have to remember that this genomic region is only half South Asian. The other half is European.
But in any case, one question that some people brought up: perhaps MacArthur has Romani heritage? I’m skeptical of this partly because:
1) there weren’t that many Romani in Britain in the 19th century
2) The British Romani are already very highly admixed
Another friend, who is a population genomicist himself, expressed some skepticism that such a long segment wasn’t broken up by recombination over the generations. My only moderately informed answer is this: we’d only notice the long segments, because if a very small region of ‘exotic’ ancestry was embedded within the dominant ancestral component it probably would not show up on some of these tests (or, we’d assume it was noise). Dan has another segment of South Asian ancestry, but much smaller in size. It may be there are other regions which we could find if we used better reference populations.
Here’s what I tentatively want to do with Dan’s data now. First, take the 80 MB or so which has South Asian ancestry, and phase it. That way I’d have a South Asian chromosome and a European one, and we could look for matches for only the South Asian one. But being busy I didn’t have time to do this. What I did have time to do was reduce the chromosomal region under consideration, and then run an IBS distance analysis in a private data set I have. This is a crude, but not always uninformative analysis. But by looking at the relationships I can now conclude that Dan MacArthur probably does not have Romani ancestry. Why? Because the Romani are of Northwest Indian heritage, and MacArthur’s match pattern using the diploid genotype (so South Asian + European) does not match what I expect would emerge from such a combination.
The full table is below, but to me the fact that he has so many matches with Northwest Indian populations is evidence that his ancestry was not Northwest Indian. Otherwise, he would be matching more Utah white (CEU samples) more often. Rather, someone with a mix of more conventional South Asian ancestry and European ancestry often resembles some of the less South Asian populations of South Asia (e.g., Brahui) in these crude measures. In fact, one of the closest matches to Dan’s IBS profile’s is that of my own mother. She is a rather vanilla ethnic Bengali, so I think there is a strong chance that his Indian ancestry is similar. This weak genetic data isn’t really the primary reason. The British East India company operated out of Bengal for much of its history, and there are simply a lot of Bengalis.
There’s a lot more that can be done here. Since I don’t have time, here’s the pedigree file if anyone wants to play with them (Dan is DGM001).
|Population||Genetic distance from Dan||Standardized distance|