Cultural transmission of a sign language when deafness is caused by recessive alleles at two independent loci (Q1324997)

Two unlinked autosomal loci are assumed to affect the ability to hear in such a way that homozygosity for the recessive allele at either locus causes deafness. The five deaf genotypes are subject to the same negative selection due to a lower likelihood of marriage, but unmarried deaf persons remain socially active and participate in the cultural transmission of sign languages. Marriages are assortative for deafness or for hearing, and mutation occurs irreversible from the dominant to recessive allele at each locus at the same rate. At mutuation-selection balance, the fully polymorphic equilibrium is symmetrical. Based on this genetic model, we consider the relative importance of various forms of cultural transmission as they affect the persistence of sign languages. Horizontal tranmission is shown to be effective when deaf children are able to interact with many peers. It is necessary, however, that some form of cultural transmission occurs between generations. Thus, vertical transmission is a critical factor, despite the fact that parent- child transmission is often interrupted due to the recessive inheritance of deafness. In particular, the contribution of vertical transmission is enhanced by assortative mating for deafness.