Assortative mating on complex traits revisited: Double first cousins and the X-chromosome.

Mate choice through direct assortment on heritable traits, assortative mating (AM), is predicted in theory to inflate the genetic variance in a population and the correlation between relatives. Here, we revisit the theory of AM, first established in the landmark 1918 paper from RA Fisher, and provide new theory and analytical results. In particular, we shed light on inconsistencies in the literature regarding the correlation between double first cousins under AM and provide a solution. We derive new theory for AM due to X-chromosome loci. We show in the latter case that the inflation of genetic variance induced under AM is twice as large in females compared to males. These two theoretical contributions are verified and illustrated through simulations. We also provide a more general unified framework for the correlation between relatives in a non-inbred population.