Inbreeding is the biological consequence of reproduction between closely related individuals. It results in an increase in the number of homozygous sites within genomes and a decrease in genetic diversity. This can reveal recessive deleterious alleles associated with genetic diseases, decrease fertility and impede the adaptive response of individuals. In humans, two strategies can limit inbreeding. First, individuals can migrate out of their native group and mate inside a new group, which corresponds to geographic exogamy. Second, in the absence of dispersal, individuals can mate within their groups according to specific matrimonial rules.
In Inner Asia, multiple human populations with contrasted social organisations and different levels of geographic exogamy cohabit. This area therefore represents an interesting opportunity to test for the presence of inbreeding avoidance strategies. In this study, we collected both ethnological and genomic data for 369 men and 177 women in 18 populations from Inner Asia (Uzbekistan, Tajikistan, Kyrgyzstan, Siberia and Mongolia). This allowed us to detect the presence of geographical exogamy for each couple and to estimate the genetic inbreeding of each individual.
First, based on genetic estimates, all populations are less inbred than under random mating, suggesting they all have some strategies to avoid inbreeding. Second, we found that the proportion of exogamous couples was highly variable between populations, from 0% to 72%. Furthermore, we found that the endogamous populations are less inbred than the exogamous ones. However, mostly or entirely endogamous populations are organized under a cognatic society while mainly exogamous populations are patrilineal. Social organization (patrilineal or cognatic), correlated to differences in dispersal behaviors, seems to lead to different patterns of genetic inbreeding.