Is Assortative Mating the Cause for the High Frequency of Connexin 26 (Cx 26) Deafness?

Recessive mutations at Cx 26 are known to be the cause of nearly half of all genetic deafness in many populations. Since assortative mating is known to increase the variance of continuous traits and the frequency of qualitative genetic phenotypes (such as recessive deafness), it seems possible that the high frequency of Cx 26 deafness may be related to the mating structure of the deaf population. During the past 200 years, improvements in educational, social and economic circumstances have lead to an increase in the fertility of the deaf along with the appearance of assortative mating in many western populations including the United States. The new deaf by deaf mating pool has increased the frequency of common forms of recessive deafness such as Cx 26 because the non-complementary matings which produce many of the deaf offspring from these marriages are proportional to the fourth power of the gene frequency. Existing data on the frequency of non-complementary marriages among the deaf in the 19th century suggest that the incidence of Cx 26 deafness has increased in the past 100 years along with the proportion of deaf children bom to deaf parents. Current observations also suggest that the high frequency of Cx 26 deafness may be confined to populations with a long tradition of intermarriage among the deaf. In Japan, for example, Fuse et al (Neuro report 10:1853, 1999) found Cx 26 deafness in only four of 20 multiplex sibships (20%). The comparable rate for multiplex probands in the U.S. is 49% (Green et al, JAMA 281:2211, 1999). Although we are not aware of data on the frequency of deaf by deaf matings in Japan, they were virtually unheard of in India in the past, and accounted for only about 1% of the marriages of the deaf in a reported survey from China (Liu et al, Chinese Med Genet 5:193, 1988). All of these findings are consistent with the hypothesis that variation in the mating structure may have contributed to differences in the current distribution of this trait throughout the world.