Biology Reference
In-Depth Information
of the parameter estimates that were observed by Rose (1975) in her seg-
regation analysis of the Fay data are given. In that large, unselected, nation-
wide sample of 1,299 DxD matings, Rose estimated that only 4.2% were
non-complementary. It seems reasonable to assume that these marriages
were random with respect to the cause of deafness. If it is assumed that
every case of non-complementation resulted from deaf parents who were
both homozygous for mutations at the
Cx26
locus, the maximum possible
frequency of the
Cx26
phenotype in the deaf population would simply be
the square root of 0.042, or about 20.1%. Molecular testing in a small con-
temporary sample of 16 apparently non-complementary matings suggests
that currently only 76% rather than 100% involve
Cx26
mutations, and
there is reason to believe that fewer were attributable to this cause in the
past. Thus, 17.8% would appear to be a conservative estimate of the
maximum probable frequency of the
Cx26
phenotype in the deaf popula-
tion during the 19
th
century.
Current estimates, using molecular testing, suggest that at least 36% of
probands referred to clinics for evaluation of congenital sensorineural
hearing loss have
Cx26
deafness, as do nearly 50% of probands from mul-
tiplex sibships (Green et al. 1999). It seems likely that the longstanding tra-
dition of intermarriages among the deaf in this country is the explanation
for the apparent doubling of the frequency of
Cx26
deafness during the past
100 to 200 years (Nance et al. 2000). It is reasonable to assume that in pre-
vious millennia the genetic fitness (i.e., relative fertility) of individuals with
profound prelingual deafness must have been very low, perhaps approach-
ing zero. Under those circumstances, virtually all new cases of deafness
would have been born to hearing parents. During the last two centuries, the
social, economic and educational circumstances of the deaf have begun to
improve. As mentioned this trend has been accompanied by an increase in
the fertility of the deaf, along with the onset of a substantial degree of assor-
tative mating in many, but not all, populations.
It is widely recognized that, for continuously distributed genetic traits
such as stature, the tendency for like to marry like has increased the vari-
ance or variability of the population beyond what it would be if marriages
occurred at random with respect to stature (Fisher 1918). For a qualitative
genetic trait such as deafness, the effect of assortative mating is to increase
the frequency of the phenotype. In the limiting case, if deafness were deter-
mined by recessive genes at a single locus, and if all deaf individuals married
one another, deafness would double in frequency in the first generation
after the onset of assortative mating. The frequency would then continue
to increase until the incidence of deaf homozygotes began to approach the
gene frequency in the population. This is the potential consequence of con-
tinued intermarriage among the deaf, which was of concern to A.G. Bell
(Bell 1883).
Because of the extreme degree of genetic heterogeneity known to be asso-
ciated with deafness, it has always been assumed that any effect of assorta-
