Biology Reference
In-Depth Information
has occurred in the family. One of the important goals of the genetic
evaluation and counseling of the parents of deaf children has always been
to try to identify the isolated genetic cases by clinical, genetic and (now)
molecular criteria. Cases that were known a priori to be sporadic would
not in general require molecular testing.
The existence of assortative mating among the deaf provides an alterna-
tive strategy for obtaining a robust estimate of the maximum frequency of
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hearing loss in the deaf population. From the distribution of deaf and
hearing offspring in DxD matings, segregation analysis permits estimation
of the proportion of these marriages that can only have deaf children (non-
complementary matings), the proportion that can only have hearing chil-
dren (complementary matings), and the remaining proportion capable of
producing both deaf and hearing children. The non-complementary matings
reflect marriages between individuals who are homozygous for recessive
alleles at the same locus, and can therefore only produce deaf offspring. The
complementary matings include marriages between individuals with non-
genetic deafness, nongenetic deafness and recessive deafness, or different
types of recessive deafness. Finally, the segregating matings include off-
spring with dominant or pseudodominant phenotypes. In Table 4.2, some
TABLE 4.2. Statistics and Parameter Estimates from
Segregation Analyses
a
Fay Survey: Proband Matings
Number of fertile Deaf ¥ Deaf matings
1,299
Total number of offspring
3,487
Nonsegregating sibships
0.831
All normal offspring (h)
0.789
All deaf offspring (y)
0.042
Segregating sibships (1-h-y)
0.169
Segregation ratio
0.325
Fay Survey: Proband Sibships
Number of sibships
2,313
Total number of offspring
13,864
Proportion of genetic cases
54.9%
Proportion of dominants among genetic cases
12.0%
Segregation ratio for dominant cases
0.260
National Survey (1970)
Total number of informative sibships
12,661
Total number of deaf offsping
16,482
Total number of offspring
49,765
Total number of deaf offspring
16,471
Sibships with deaf parents (D¥D)
Total number of sibships
421
Total number of children
1,356
Deaf children from non-complementary matings
451
Deaf children from segregating matings
538
a
Taken from Rose (1975)
