Turns out I am genetically British and not Gallish!

On January 28th 2014, I became a British citizen (while still retaining my original French citizenship). This blog post is not about the reasons behind that decision, that I might or might not expose someday. But I just discovered that I might already be British by descent!

A year ago, I sent some spit to a US company called 23andMe. This company amplified my DNA and looked for all the sites that are variable in the human population (e.g. some people present at a given position on a chromosome the base pair G-C while other have the base pair A-T). We called those SNPs (for Single Nucleotide Polymorphisms, pronounce “snips”), and the procedure SNP genotyping. My aim was to look for increased risks for some diseases. If we have the genotype (the state of all the SNPs) of many people, patients and controls, one can determine if some SNPs are statistically more frequent in people with a disease than in the general population. I thus discovered that I had a higher risk than the general population of having Venous Thromboembolism, Alzheimer’s Disease and Exfoliation Glaucoma. Conversely I have a lower risk of Gout, Colorectal Cancer and Age-related Macular Degeneration (so less chance of becoming blind with macular degeneration but more with glaucoma, hmm). I have also a higher sensitivity to the blood thinner warfarin and a decreased sensitivity to treatments for type II diabetes and hepatitis C. This health related activity of 23andMe was recently blocked by the Food and Drug Administration, a decision that led to many interesting and heated discussions (google “23andMe” “FDA” to have an idea of the divide that decision brought to the community of biologists).

However, there is another use of SNP genotyping: genealogy. Because mutations are very rare, most SNPs tend to be conserved between parents and children. As a result, I share around 50% of my SNPs with each of my parents and my children (49.9% with my daughter who was also 23andMeed), 25% with my grand-parents etc. We have a lot of SNPs, several millions. That means we can trace genealogical relationships very far. If we know the geographical localisation of the 23andMe clients, and assume most of them did not travel away from where their ancestors lived, one can use the genotyping to follow genetic migrations.

23andMe offers tools to do just that, and surprise … I am mainly of British ancestry! One talks here of the ancient ancestry, before fast travels and genetic mixing. 23andMe provides three levels of statistical analysis, from the most conservative (less informative, but more robust) to the most speculative (more informative but less solid).




Whatever level of significance, what stands out is “British & Irish”. Another tool allow to visualise this DNA ancestry on a global Similarity Map.


Each square represent a genotype, i.e. a person, significantly related to me. My genotype is represented by a green “callout”. My contacts on 23andMe (people I contacted or who contacted me, because we are perhaps distant relatives) are represented by black callouts. As you can see on the maps, my contacts and I cluster better with the British cloud than the French one.

What could that mean? I can imagine three reasons, with increasing likelihood.

1) Most of British clients of 23andMe are of French ascent. Therefore the apparent result is the opposite of the truth, the cluster labelled “British” being made of French genotypes. This is not completely insane. After all, William the conqueror and his army came from France. However, him and his army were norman, i.e. viking (and since Normandy is close to Britanny, that could rather explain the 1.3% scandinavian).

2) There was a recent influx of British DNA in my ancestry lines. This is certainly possible, but more than 50%? The influx should be recent and multiple (affecting both lines of ancestry). And indeed looking at the standard and speculative mapping above, we see British ancestry on both sets of chromosomes.

3) Some theories postulate that invasions do not affect the general population, only aristocracy being replaced. They received support from genotyping. Principal component analysis of genotypes from a pan-European cohort reproduced the geographical origin of its members. In other words, the genotypic distances followed the geographical distances.

Is-it always true though? Both my parents are “bretons“, coming from Britanny. Britanny was invaded repeatedly between the 3rd and 6th century by britons fleeing picts and anglo-saxons. Some legends state that these britons killed celtic mens and fecundated the women (the same legends state that the women’s tongues were cut so that they could not teach their own dialect to their children). Could it be that in Britanny the genetic pool was genetically replaced?

I would be very interested to hear from any other breton having had their genotype done.


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