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for Cx26 deafness is becoming more widely available in the deaf commu-
nity, it is useful to assess the range of the potential effects that positive or
negative marital selection for Cx26 genotypes might have on the overall
incidence of deafness. Using data on the overall frequency of deaf children
who are born to deaf parents (Table 4.2) and data on the frequency of pseu-
dodominance and Cx26 deafness, it can be estimated that the complete
avoidance of at-risk Cx26 marriages among the deaf would lead to approx-
imately a 2% reduction in the overall incidence of deafness. Conversely,
complete genotypic assortment for Cx26 deafness would lead to approxi-
mately an 8% increase in the first generation. The long-term effect of con-
tinued complete genotypic assortment would be the progressive reduction
of heterozygotes in the population until the frequency of homozygotes
began to approach the gene frequency of about 1.5% (Green et al. 1999).
In comparison with phenotypic assortative mating, genotypic mate selec-
tion would greatly accelerate the approach to this limit. Nevertheless,
despite the extreme nature of the alternative assumptions, the immediate
effects of genotypic mate selection are relatively modest.
5.4 Estimating the Frequency of Common Forms
of Deafness
Clearly, the frequency of a common form of hearing loss such as Cx26 deaf-
ness can differ among racial or ethnic groups. Since it is a genetic form of
deafness, the incidence will also differ in probands from simplex and mul-
tiplex sibships; and because of the mating structure of the deaf population,
the incidence will even differ in the deaf offspring of deaf and hearing
couples. For these reasons, obtaining reliable estimates of gene and geno-
type frequencies will require a random sample that includes proportionate
representation from all relevant subgroups of the deaf population, or at
least knowledge of the distribution of these subgroups, so that overall esti-
mates can be reconstructed from stratified samples.
5.5 Other Examples of Nonrandom Mating
Phenotypic assortative mating is not the only form of nonrandom mating
that can alter the frequency of genotypes from those expected under Hardy-
Weinberg equilibrium. Inbreeding increases the frequency of all rare reces-
sive phenotypes above the values expected with random mating. Similarly,
were it not for racial and ethnic homogamy, the frequency of sickle cell
anemia and Tay-Sachs disease (at conception) in this country would be far
lower than the current incidence. These two diseases are useful examples
for those who might be alarmed by the increase in the incidence of Cx26
deafness that appears to have occurred in this country during the past
century. Marriages among the deaf are an integral part of a culture that has
greatly enriched the lives of both the deaf and hearing segments of society
during the past two centuries. Unless we are prepared to advocate the
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