Deaf intermarriage does not increase the prevalence of deafness alleles

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 Science. Bells hypothesis was not supported by a 19th century study by Edward Allen Fay, which was funded by Bells 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 an apparent increase in non-complementary pairings when a recent dataset of Gallaudet alumni was compared with the 19th century Fay dataset, Bells argument has been resurrected that residential schools for the deaf, which concentrate signing deaf individuals together, have promoted assortative mating and increased the prevalence of both phenotypic deafness and the commonest recessive deafness allele. Because this hypothesis persists, even though it contradicts classical models introduced by R.A. Fisher and Sewell Wright, it is critically important that this hypothesis be thoroughly re-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, operated without assumptions of parametricity, and captured ending distributions and variances. Our simulation results affirm predictions from classical equations and show that assortative mating only modestly increases the prevalence of phenotypically deaf individuals, with this effect mostly completed by the third generation. Most importantly, our data show that even intense assortative mating does not increase allelic frequency under reported conditions. These results are not locus-specific and are generalizable to other forms of recessive deafness. We offer alternative explanations for the higher rate of non-complementary pairings measured in the contemporary Gallaudet alumni sample as compared to the Fay dataset.