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which allow outer hair cell cochlear function to be assessed somewhat
independently of inner hair cell and neural function. Recent studies suggest
that emissions may be helpful in discriminating between those who carry
certain mutations that cause recessive nonsyndromic sensorineural hearing
impairment and those who do not (Morell et al. 1998). Additionally, by per-
forming tests for middle ear muscle reflexes, ABR, and otoacoustic emis-
sions, a patient with hearing loss caused by abnormal hair cells can be
distinguished from one with normal hair cells (Berlin et al. 1996). Patients
with normal outer hair cells do not usually benefit from hearing aids in the
frequency ranges where their hair cells, and presumably their emissions, are
normal.
It is important to remember that the etiology of hearing loss may not be
the same, even for members of the same family. One or more members of
a family may have an acquired hearing loss that mimics that of members
with genetic hearing loss. Such phenocopies in a family are common if the
only test that is done for phenotyping is a pure tone audiogram. This prac-
tice should be discouraged, and all family members should have at least
tympanometry, middle ear muscle reflexes, otoacoustic emissions, speech
audiometry, pure tone audiometry by air and bone, and speech discrimina-
tion testing, if at all possible.
A poignant example (uncovered as part of our genetic studies) is a family
in which several members were reported to show marked hearing losses
based on pure-tone audiometry. However, by adding tympanometry,
reflexes, emissions and speech audiometry to the phenotyping battery, one
of the patients, a 36-year-old female who had been wearing hearing aids
since she was 7 years old, volunteered a pure-tone average hearing loss of
65 dB, but had normal otoacoustic emissions, normal reflexes, a Speech
Reception Threshold of 20 dB, and 92% discrimination at only 40 dB HL.
Experienced audiologists recognize the anomaly of a 65 dB hearing thresh-
old with a 20 dB Speech Reception Threshold. The patient's hearing aids
were in her ears, but were never turned on. Further inquiry revealed that
she had been hospitalized for conversion hysteria, and she admitted to
adopting the hearing loss so that she “would belong.” As discussed later in
this chapter, incorporating this individual into a linkage study based on her
pure tone audiogram would have resulted in inaccurate conclusions.
The audiologist needs to be sensitive to the importance of accurate and
comprehensive phenotyping and is urged to obtain more than simple pure-
tone audiometry to assist the geneticist in both research studies and clini-
cal service.
1.2 Inheritance Patterns of the Phenotype
The human genome consists of 24 different types of chromosomes in the
nucleus of a cell. They are designated 1-22 (autosomes), X and Y (sex chro-
mosomes). An offspring inherits one set of chromosomes from each parent,
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