Biology Reference
In-Depth Information
deafness in the parents, children and siblings of 4,471 marriages occurring
between 1803 and 1894 in which one or both of the partners were deaf (Fay
1898). Overall, Fay found that only 9.7% of these marriages produced deaf
offspring. Although this frequency was far greater than the incidence
among hearing couples, he also emphasized that these marriages were “far
more likely to result in hearing offspring than deaf offspring.” Fay also
showed that marriages involving parents with “congenital” as opposed to
“adventitious” deafness appeared to have a higher incidence of deaf off-
spring and, in his discussion of the effects of consanguinity, Fay provided
further evidence that he recognized the heterogeneous nature of deafness.
In his monumental study
Marriages Among the Deaf in America
, Fay (1898)
wrote:
“Now if a person who is deaf from one anomaly or disease of the auditory organs,
or of the nervous system marries a partner who is deaf from some different and
unconnected pathological condition, the law of heredity under consideration should
not lead us to expect any intensification of the liability to transmit the defect, for
the characteristics existing in the two partners are not the same. Although both part-
ners are deaf, these marriages are not a union of 'like with like' from a physiologi-
cal point of view, and the law does not apply to their case. On the other hand, where
the physical condition that results in deafness is the same in both partners as for
instance it probably is in consanguineous marriages of deaf persons—we should
expect the liability of deaf offspring to be intensified by their union.”
Fay was able to identify a small sample of 31 consanguineous marriages
among the deaf. He found that 45% of the marriages had at least one deaf
child, and that 30% of the offspring from these marriages were deaf. Fay
concluded that consanguineous marriages were very likely to have deaf off-
spring whether the deafness was congenital or adventitious, whether there
were other deaf relatives, and whether only one or both parents were deaf.
Without the benefit of Mendel's yet to be rediscovered theory of the gene,
Fay's intuition about the significance of consanguinity is remarkable and
anticipates the extensive use of such marriages to map genes for deafness
by nearly a century.
2.2 Post-Mendelian Era
Following the recognition of Mendel's work, the Fay data set has been
repeatedly reanalyzed using progressively more complex genetic models. In
1910, Hammerschlag, who was familiar with the transmission of recessive
deafness in the mouse, attempted to explain human deafness by a recessive
gene at a single locus. However, even when the data set was limited to deaf
by deaf (DxD) matings in which each parent had three or more deaf sib-
lings, he found that more than half the offspring were hearing, instead of
the expected 100% deaf. Later, Kraatz (1925) proposed a two-locus model
in which deaf individuals were assumed to be homozygous at one or both
