Genetic aspects of Huntington's chorea: results of a national survey.

In a nationwide survey of South Africa we ascertained and investigated 157 persons with Huntington's chorea and collected data on a further 328 deceased individuals who had probably been affected. Of the 157 respondents, 17 had a juvenile onset and in 13 of these the father had been the affected parent. Further analysis of these data shows that the father had transmitted the gene to 5 of the 6 children with onset in the first decade. The heterozygote frequency in the combined white population and the group of mixed ancestry is 6.7 X 10(-5) and the minimum number of heterozygotes is 458 in South Africa (1 in 13,000). The disorder is extremely rare in African Negroes; only 3 affected individuals were identified out of a population of 19 million.