Estimation of genetic load in cases with profound congenital non-syndromic hearing impairment

Hearing impairment (HI) is a frequently occurring sensory defect, causing physical disability among the affected. Earlier studies revealed that hereditary HI affects about 1 in 1,000 children in most populations. It is a highly heterogeneous trait with 85% of the cases being autosomal recessive and hence occurs more frequently in populations where the parents of affected are blood relatives. The present study comprising 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 indicates the involvement of a recessive component in the expression of the condition. The mortality rate among the sibs of probands was high in the inbred group (21.95%) as compared to the non-inbred group (12.06%), indicating the segregation of detrimental recessive genes along with genes 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 studied was segregational. (C) 1998 Lippincott-Raven Publishers.