Biology Reference
In-Depth Information
in the cochlea is less well understood, it is known to be present in the base-
ment membrane of the stria vascularis. Alpha tectorin is an important com-
ponent of the tectorial membrane that supports the hair cells. Precisely how
mutations in this gene cause deafness is not yet clear. In a Swedish family,
Balciuniene (1998; 1999) found evidence for linkage of hearing loss to the
alpha tectorin locus on 11q22 and also to a second locus,
DFNA2
, on chro-
mosome 1p35.1. Subjects with both mutations had more severe hearing loss
than those with single mutations at either locus, suggesting an epistatic
interaction between the two loci. The C1057S substitution in the tectorin
protein may have predisposed to abnormal crosslinking of the polypeptide.
5. Mating Structure of the Deaf Population
Assortative mating is a distinctive feature of the genetic architecture of deaf
populations in many countries. One potent effect of this pattern of mate
selection is to bring together rare genes at different loci that would other-
wise have a low probability of coexisting in the same individual. Although
90% of deaf individuals in the United States marry deaf partners (Schein
and Delk 1974), linguistic homogamy (shared manual communication),
rather than phenotypic assortment for deafness, may in fact be the basis for
mate selection. The observation suggesting this may be the case is the
fact that the hearing partners in deaf-by-hearing matings are often “native
signers” who are themselves the offspring of deaf couples. Despite the fact
that they are not deaf, these individuals may carry genes for recessive deaf-
ness at multiple loci, and matings of this type can sometimes give rise to
pseudodominant transmission of deafness.
5.1 Frequency of Common Forms of Deafness
The discovery that mutations in a single gene (
connexin 26
), are the com-
monest cause of genetic deafness was unexpected. Estimates of the relative
frequency of
Cx26
deafness have varied greatly with up to 50% of all child-
hood deafness being attributed to this cause in some populations (Steel
1998). In others, such as India, Japan, Mongolia and China, the incidence
appears to be much lower. Most reported studies have been based on
molecular testing of clinic populations, and have not in general involved
the random or stratified random sampling of subgroups likely to exhibit
different frequencies. Adding to the confusion, probands with no affected
siblings have in some reports been designated sporadic cases. The term spo-
radic refers to cases of deafness in which there is a very low chance of recur-
rence within the family, comparable to the incidence of deafness in the
general population. Most sporadic cases of deafness are caused by envi-
ronmental etiologies, but some can also represent new dominant mutations.
Although probands with no affected siblings may be sporadic cases, they
may also be isolated genetic cases in which by chance only one deaf child
