|
Genetic factors contribute to more than half of all congenital hearing losses and are also responsible for later-onset hearing losses. Understanding the factors underlying hereditary hearing loss requires locating the genes responsible for hearing loss and defining the specific mechanisms and functions of those genes. From a clinical standpoint, this information may contribute to improved management strategies for individuals with hereditary hearing loss and their families. Accurate determination of the auditory characteristics associated with various genetic abnormalities requires the use of measures sensitive to subtle aspects of auditory function.
Hereditary Hearing Loss
Congenital (hereditary) hearing loss occurs in approximately 1–2 of 1000 births, and at least 50% of all cases of hearing loss have a genetic origin (Morton, 1991). Although hereditary hearing losses may occur in conjunction with other disorders as part of a syndrome, the majority of cases are nonsyndromic. Later-onset hereditary hearing loss occurs at various ages from the first decade to later in life.
Chromosomes
Human cells contain 23 pairs of chromosomes (22 pairs of autosomes and two sex chromosomes). Hereditary material in the form of DNA is carried as genes on chromosomes. Cells reproduce by mitosis (meiosis for the sex chromosomes), where chromosomes divide, resulting in two genetically similar cells. Errors can occur during mitotic or meiotic division, resulting in cells with chromosomal abnormalities and an individual with a chromosomal defect.
Genotype and Phenotype
Genotype describes an individual's genetic constitution. Phenotype relates to the physical characteristics of an individual and can include information obtained from physiological, morphological, and biochemical studies. Auditory tests contribute to the phenotypic description.
| 
