The incidence of hearing impairment in India and Pakistan is higher than in the United States. This can be attributed to several factors, including infection and high rate of inbreeding which may result in homozygosity for a rare recessive mutation. The average family size is larger in India and Pakistan when compared to the United States. In addition, there are more than 1 billion people in India while the population size of Pakistan is approximately 148 million. The ability to detect linkage is greatly enhanced in an extended inbred family. It is estimated that as many as 45% - 60% of marriages in Pakistan are between close relatives (the vast majority of marriages are to first cousins). While the percentage of consanguineous marriages are not as high in India as compared to Pakistan, the population size of India is much larger.
Hearing impairment is the result of abnormal ear development, abnormal ear function or both and yet little is known about the molecular mechanisms involved in the development and homeostasis of the inner ear. Although the genes for several recessive deafness loci have been identified, there are still many families segregating deafness that cannot be ascribed to one of the currently known genes. The purpose of this study is to continue to identify genes that cause nonsyndromic hereditary hearing impairment by enrolling families segregating deafness.