Blood groups in ancient Europe

This high frequency in Europe has long been surprising because the Rh- phenotype has an obviously deleterious effect. In particular, if an Rh- mother has an Rh+ child (because the child inherited a Rh+ allele from its father), then it’s possible for the mother to produce antibodies against the Rh+ antigen leading to haemolytic disease and severe illness for the child. There are three common explanations for the high frequency of the Rh- allele. First, there might be some (unknown) beneficial effect of the Rh-allele. Effects like this related to malaria resistance are what drive the high frequencies of the sickle cell trait and many thalassemias. On the other hand, no obvious selective advantage is known, there are no obvious genomic signals of selection on the Rh- allele, and if there were a selective advantage to the Rh- allele then we might ask why it hasn’t fixed, since once the Rh- frequency rose above 50%, it would be selected for, rather than against. So this only really works if it is overdominant. Another possible explanation is reproductive overcompensation – Rh- women have more children to replace the ones that die for haemolytic disease. But this seems unlikely. The range of parameters for which this model works is relatively small, particularly since the effect gets worse for subsequent pregnancies. A third explanation, suggested by Haldane in 1942 is that the high frequency in Europe is due to the fact that present-day Europeans are recently admixed between a population that has a very high frequency of the Rh- allele and one that a very low frequency. In fact, the ancient DNA evidence suggests that something like this is close to the truth. Cavalli-Sforza and colleagues thought that this was specifically a mixture between Rh- hunter-gatherers and Rh+ Farmers, largely based on the observation that the Basque population, who they believed to have largely hunter-gatherer ancestry, have a very high frequency of the Rh- phenotype.

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