ESTIMATION OF GENETIC LOAD IN CASES WITH PROFOUND CONGENITAL NON-SYNDROMIC HEARING IMPAIRMENT

Hearing impairment (HI) is a frequently occurring sensory defect, caus ing physical disability among the affected. Earlier studies revealed t hat hereditary HI affects about 1 in 1,000 children in most population s. It is a highly heterogeneous trait with 85% of the cases being auto somal recessive and hence occurs more frequently in populations where the parents of affected are blood relatives. The present study compris ing 244 profound congenital HI cases in a hospital in Hyderabad, India , revealed a positive family history in 47 cases (19.26%), indicating a genetic basis for the condition. 42.6% of the cases showed parental consanguinity with an average inbreeding coefficient ((F) over bar i) 0.029, which is much higher than that found in the general population of Hyderabad (consanguinity 24%; (F) over bar i = 0.015). This indicat es the involvement of a recessive component in the expression of the c ondition. The mortality rate among the sibs of probands was high in th e inbred group (21.95%) as compared to the non-inbred group (12.06%), indicating the segregation of detrimental recessive genes along with g enes for CHI in the inbred families. The estimated genetic load was 2. 33 and the B/A ratio 6.96, indicating that the load among the cases st udied was segregational. (C) 1998 Lippincott-Raven Publishers.