Biology Reference
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of two loci. This model could readily explain how deaf by deaf matings
could either produce all normal (complementary matings) or all affected
offspring (noncomplementary matings), but could not readily explain the
substantial frequency of deaf by deaf matings that produced both hearing
and deaf offspring. As an alternative, Dahlberg (1931) proposed an even
more complex genetic model in which affected individuals were assumed
to be carriers, simultaneously, of dominant genes at three loci, as well as
homozygotes at another locus (to explain the increased consanguinity). By
the appropriate selection of gene frequencies, he showed how the observed
proportions of affected individuals in various mating types could be
explained. Under Dahlberg's ingenious model, the high frequency of sib-
ships with only one deaf child is explained by mating types such as (A/a,
B/b, C/c, d/D)
¥ (a/a, b/b, c/c, d/D), which have a 1/32 segregation ratio,
rather than by sporadic environmental causes. This model, however, does
not provide a mechanism for dramatic increases in the frequency of
sibships with only one deaf child, as occurs, for example, with a rubella
epidemic.
In 1956, Stevenson and Chessman collected family data on 700 probands
or affected-index cases from Ireland who became deaf before six years of
age, a sample that included virtually no recognized cases of rubella deaf-
ness. For the offspring of consanguineous matings, a recessive model that
assumed complete penetrance for the deafness given homozygosity for the
deleterious gene, provided an excellent explanation for the data. In these
families, there was no evidence for prenatal loss of deaf fetuses, or for
failure of the genotype to be expressed (reduced penetrance) because of
modifier genes or other factors. However, among the offspring of non-
consanguineous hearing by hearing (HxH) matings, there was a 25% excess
of cases that appeared to be sporadic or nongenetic. Also, 21 of 32 DxD
matings that were ascertained through the parents had hearing offspring,
and only five had deaf offspring exclusively. Because these families were
identified (ascertained) through deaf marriage partners regardless of their
children's hearing status, they included families with hearing offspring
exclusively. (In contrast, ascertainment through offspring would only have
included families with at least one deaf child). The authors recognized their
findings as an indication of the existence of two or more recessive loci.
If all deafness was caused by recessive mutations at a single locus, all the
children of deaf by deaf matings would be expected to be deaf. If deafness
was caused by two equally frequent recessive genes, and deaf by deaf
marriages occurred at random, only one half of the marriages would be
expected to produce deaf offspring exclusively. The remaining “comple-
mentary” matings between individuals with different recessive genes would
produce only hearing offspring. This hypothesis was consistent with the
frequency of consanguinity, which was much higher in Stevenson and
Chessman's (1956) sample than would have been expected if all the cases
of recessive deafness had been caused by mutations at a single locus.
