Although it’s a difficult topic to study, because the human species has experienced a number of population bottlenecks over time it’s assumed that rates of consanguinous (close kinship) marriage or partnership have been quite high at times in human history. Today, more than 10% of the world’s progeny is derived from consanguinous partnerships, especially within sub-Saharan Africa, the Middle East, and throughout parts of Asia. Children born of consanguinous parents have a 3.5% higher mortality rate, although it is difficult to tease out other effectors such as poverty .
Interestingly, however, there is some indication that consanguinous pairing may be protective for certain types of diseases as well.
One of the fascinating things that population geneticists have discovered is that smaller populations naturally tend to have less variability than larger ones (duh, right?). In human traits that are multifactorial, like autism for instance, some genetic variants can affect the expression of others. And in some cases, this can lead to a “covering up” of the effects of a genetic variant that, alone, would be harmful .
That is, until you marry someone outside of your village and– in the case of today– perhaps someone with whom you may share your last common ancestor thousands of years ago, you are unaware that one of your gene variants could be harmful. Intermixing decreases the likelihood that your partner shares the same genetic variants as you and also decreases the likelihood, thanks to sexual recombination, that your kids are going to inherit all those interacting variants together that help keep certain harmful diseases at bay.
In the simplified scenario above, a segment of homologous chromosomes are sexually recombined from the mother and father. Paired variants, A and B, are unpaired, unmasking the expression of variant B.
While we’re not proposing that we start marrying our cousins again, understanding the links between large-scale population genetics and complex conditions like autism, heart disease, and cancer can help us to understand why they may be even more common. Intermixing across what were once isolated human populations helps to prevent rare recessive diseases, but it may also increase the likelihood that small-effect deleterious genetic variants can be unmasked.