Genetics and Ancestry

This entry explores the influence of genetic and inherited causes of deafness on the distribution of deaf individuals and families within the population and, as a result, on social patterns, differing identities, and communication. It focuses on people born deaf or deaf from shortly after birth rather than on people who became deafened later. The latter group are less likely to have links to or affinity with Deaf communities and languages; also, aging, infection, or injury rather than genes are the more common causes of later-onset deafness.

In most of the world, there is a low proportion of deaf people within the population and they are geographically widely dispersed. There are small pockets where the incidence of deafness is higher and all residents, deaf and hearing, use a signed language daily: notably, in small villages in places such as Adamorobe, Ghana; Yucatan, Mexico; El-Sayed, in the Negev Desert of Israel; and Bengkala in Bali. The most famous concentration of deaf people and sign language use was on Martha’s Vineyard Island, Massachusetts. There are multiple societal factors affecting how deaf people are distributed within the majority hearing population. Some factors tend toward the establishment of larger Deaf communities, whereas others are reasons why deaf people may have less contact with other deaf people. Some of these factors are imposed on a deaf individual (e.g., placement in a Deaf or mainstream school), and others are individual choice (e.g., marriage within the Deaf community, migrating to areas with larger Deaf populations). As well as societal factors, genetic causes of deafness are another significant factor influencing the pattern of deaf individuals and families within populations. Some causes mean that many members of an extended family are deaf, whereas others make it much more likely that only one individual is deaf within a particular family.

More than 90 percent of deaf children are born to hearing parents, and only 10 percent of children born to deaf parents are deaf themselves. This statistic is quoted often but is significant when one considers how the varied combinations of deaf and hearing individuals within one family can have a profound impact on the identity and communication of the members. On average, the chances of a deaf couple having a deaf child are the same or lower than the chances of a hearing couple with one deaf child having a second child who is also deaf. These figures may seem counterintuitive, but the underlying reason is the large number of different causes of deafness. The following sections explain how genes that are inherited in different ways (modes of inheritance) cause different patterns of deafness within a family.