The risk of having a second retarded child.

We have studied segregation ratios in 282 Israeli families with normal, nonconsanguineous parents and retarded offspring without specific etiologic diagnosis. Severity of retardation and nature of medical history significantly affected recurrence risk, while sex of propositus and sibs and presence or absence of epilepsy, cerebral palsy, microcephaly, and short stature did not. Segregation rations were 0.095 in cases of severe retardation with normal medical history, 0.216 for mild retardation with normal medical history, 0.230 in cases of all retardation with maternal reproductive inefficiency, and 0.110 for all retardation with other reported prenatal, perinatal, or infantile complications. Estimated recurrence risks in most simplex families dropped sharply with each additional normal child. The recurrence risks are higher than some previously published estimates. Different ascertainment criteria may be responsible for this variation. The criteria used here were compatible with a 3.2% population prevalence of mental retardation with a 0.223 segregation ratio in multiplex sibships. It is recommended that future studies of recurrence risks include similar data permitting evaluation of sensitivity of ascertainment criteria.