The term “eugenics” has very negative connotations today. Nevertheless, in some ways society is moving in a direction which results in “eugenical” outcomes, insofar as allele frequencies and genotypes are skewed from what would otherwise be the case if natural processes operated without human volition.* Probably the most obvious case in modern medicine is the high rate of abortion of fetuses which are inferred to carry the genetic profile of an individual with Down Syndrome. In the future this sort of instance will be more general, as high quality prenatal genome sequencing along with progress toward understanding of the basis of inheritance of Mendelian diseases will avail parents of many choices. This will naturally result in a lot of discussion and debate about ethics and values.
But there is a less high-tech and ethically fraught form of eugenics, which is nevertheless culturally controversial. One of the overlooked aspects of the 2009 paper Reconstructing Indian population history is that it found that many Indian populations had an excess of homozygosity, likely due to long standing endogamous practices encouraged by the caste system. This, despite customs which enforce exogamy for Hindus across much of India, in particular the North. Within the abstract the authors suggest then that “there will be an excess of recessive diseases in India.” Recently I spoke to a young woman of Jat background whose parents are very traditional. I told her the issue relating to homozygosity, and communicated that to gain the benefits of masking genetic load one need not go genetically and culturally very far. An individual of the same ethnicity and religion would be sufficient, so long as they were not of the same caste (jati).**
This is not only a South Asian issue, as evident in a recent paper in The European Journal of Human Genetics, Genotyping of geographically diverse Druze trios reveals substructure and a recent bottleneck. Following up on earlier work it confirms some structure within the Lebanese Arab population. This should not be surprising, as the distribution of ethno-religious groups within Lebanon is not geographically arbitrary. Whether they live in Beruit today, the Maronite Christians for example often have a ancestral background from around Mount Lebanon. Additionally, the Muslim groups within Lebanon have been subject to a proportion of admixture with foreign populations over the past ~1,000 years. Nevertheless, it does seem that overall the Lebanese of all sects derive from a common ancestral group, and exhibit more affinities with each other on the whole than with non-Lebanese populations (the Druze have been subject to a stronger bottleneck than the other sects, explaining why they are distinct genetically).
In any case, the major finding in these results is that there are elvated levels of homozygosity in individuals who are Lebanese who are the product of marriages between “unrelated” parents. Additionally, individuals of Muslim background who are the product of first cousin marriages show evidence of being descended from common ancestors recently across multiple paths, suggesting a more ubiquitous practice of cousin marriage within this group. This mean that “first cousins” in the Muslim community often exhibit a relatedness greater than that of idealized first cousins because they are part of an extended inbred lineage.
In the figure to the right you can see the distribution of runs of homozygosity for different Lebanese sects comparing individuals who are the product of unrelated parents and those who are first cousin offspring (against a reference set of Europeans). You note that in the 0.5 to 1 Mb range of homozygosity tract length Europeans are highly enriched in comparison to Arab Lebanese. This is almost certainly due to the bottlenecks that Europeans have been subject to over the past ~50,000 years. Most Middle Eastern populations a priori should have a higher long term effective population size assuming a serial founder effect, as well as the ecological context of the Ice Ages. But at 1-2 Mb you see that long term inbreeding shows up in all the Lebanese groups. Notice that the further you go up in tract length the more unequal the ratio between the first cousin offspring (FCO) and those who are the products of “unrelated individuals” (URO). This is what you expect. But even at greater than 16 Mb you see that Lebanese who are unrelated still have many more of these segments than Europeans. This is a major tell that cryptic relatedness is a bigger issue within the Lebanese population than among Europeans.
The recommendation here then would be simple: Lebanese Arabs should marry individuals who are culturally aligned with them (e.g., Greek Orthodox Lebanese marrying Greek Orthodox Palestinians, Maronites with individuals from other Arab Christian groups in alignment with Rome, etc.), but have a different genetic history. This shouldn’t be controversial, but it can be (e.g., warning about the dangers of cousin marriage in terms of birth defects has resulted in accusations of Islamopobhia in Britain because of the power of identity politics in a multicultural society). Yet to some extent I believe that first cousin marriages will decline throughout the Middle East because of the demographic transition. If you don’t have many first cousins, then the potential for first cousin marriages declines greatly. But as I noted above in many Indian Hindu groups exogamy is normative, but there is still an issue with elevated homozygosity, so even without first cousin marriages there could be some gains in utility on the margin from outbreeding further than just near relatives.
Finally, aside from the straightforward genetic issues, there may be social benefits to the breakdown of clan structures which are the driving force behind population wide elevated homozygosity. Keeping it “all in the family” may not be conducive to broader national trust and cohesion. Readers probably know enough about the modern Middle East so that I don’t need to elaborate on this issue angle….
* In a narrow sense eugenics should actually result in allele frequency changes, but in modern practice this is not always the case. E.g., the screening for individuals with Tay-Sachs carrier alleles changes the genotype frequency in the population so it is not in Hardy-Weinberg Equilibrium.
** Another thing that might be feasible to be to attempt to infer potential enrichment of homozygosity by looking at genotypes of pairs of potential mates within the same caste.
Comments are closed.