Deaf intermarriage does not increase the prevalence of deafness alleles

Abstract The idea that deaf intermarriage increases deafness was forcefully pushed in the late 19th century by Alexander Graham Bell, in proceedings published by the National Academy of Sciences. Bell’s hypothesis was not supported by a 19th century study by Edward Allen Fay, which was funded by Bell’s own organization, the Volta Bureau. The Fay study showed through an analysis of 4,471 deaf marriages that the chances of having deaf children did not increase significantly when both parents were deaf. In light of what appeared to be an increase in non-complementary pairings when a modern dataset of Gallaudet alumni was compared with the 19th century Fay dataset, Bell’s argument has been resurrected in the published genetics literature that residential schools for the deaf, which concentrate signing deaf individuals, have promoted assortative mating based on language compatibility. A published report from a computer simulation posits that assortative mating could have increased the frequency of deafness alleles. Because this hypothesis, and the results of that computer simulation, contradict classical models of inbreeding introduced by Fisher and Wright, it is critically important that this hypothesis be thoroughly investigated. In this study, we used an established forward-time genetics simulator with parameters and measurements collected from the published literature. Compared to mathematical equations, simulations allowed for more complex modeling, were free of assumptions of parametricity, and captured ending distributions and variances. Our simulation results, whose median results matched predictions from mathematical equations, show that assortative mating modestly increases the incidence of phenotypically deaf individuals with this effect mostly completed by the third generation. Most importantly, our data show that assortative mating does not affect allelic frequency under reported conditions. We offer alternate explanations for the higher rate of non-complementary pairings measured in the contemporary Gallaudet alumni sample as compared to the Fay dataset.