Consanguinity analysis in Israeli mental retardates.

Consanguinity rates were analyzed in 904 families of retardates studied in 11 Israeli Jewish ethnic groups. It was estimated that the representative recessive gene frequency is .00518, implying that a gene equilibrium maintained by mutation alone is improbable and that some other hypothesis should be considered. The proportions of homozygotes among the following idiopathic subgroups are estimated as follows: 18%-19% homozygotes among severe idiopathic retardates with nonconsanguineous parents and no affected siblings; 74%-76% homozygotes among severe idiopathic retardates with first-cousin parents and no affected siblings; 5% homozygotes among mild idiopathic and idiopathic-familial retardates with nonconsanguineous parents; and 41% homozygotes among mild idiopathic and idiopathic-familial retardates with first-cousin parents. The estimated number of major gene loci within ethnic groups is 17-21 for severe idiopathic retardation and 43-61 for mild idiopathic retardation. These findings provide a basis for genetic counseling of families with single retardates of unknown cause. They can also be useful in epidemiologic studies of nongenetic factors. The great prevalence of common gene defects causing retardation, coupled with the rarity of disorders of amino acid metabolism in the same series, seem to indicate that further emphasis on amino acid metabolism may be nonproductive in the scientific study of retardation and that other biochemical approaches should be encouraged.