Biology Reference
In-Depth Information
FIGURE 6.7. Type I Waardenburg syndrome (from Asher and Friedman 1990).
Characteristic WS1 features are present including dystopia canthorum, heterochro-
mia irides, and a white forelock. (Reproduced from Asher and Friedman, Journal
of Medical Genetics 27:617-626, Copyright 1990, with permission of the BMJ Pub-
lishing Group.)
(Marcus 1968). Iris pigmentation anomalies may include eyes of different
colors, brilliant blue eyes, or two segments of different colors within a
single eye. The hair, eyelashes, or eyebrows may be prematurely gray, or
there may be a white forelock (Farrer et al. 1992) (Fig. 6.7). The diagnostic
criteria for WS1 and WS2 have been enumerated, tabulated and reviewed
(Farrer et al. 1994; Farrer et al. 1992; Liu et al. 1995; Read and Newton
1997).
There is reduced penetrance of the sensorineural hearing loss phenotype
in WS1 and WS2, and within and between families the hearing loss is highly
variable in severity, symmetry, onset, course, and in the observed audio-
metric pure-tone pattern (Fig. 6.2) (Newton 1990). Waardenburg estimated
the penetrance for hearing loss at 20% (Waardenburg 1951). More recent
estimates of penetrance for hearing loss are 36 to 58% for WS1, and as high
as 87% in some WS2 families (Liu et al. 1995; Morell et al. 1997; Newton
1990). There are some large WS families with even higher penetrance for
deafness (da-Silva 1991; Morell et al. 1992), strongly indicating the existence
of modifying factors in addition to the possible effects of stochastic events
(Morell et al. 1997; Pandya et al. 1996; Read and Newton 1997).
