Affected parent and age of onset in Huntington's chorea.

In a recent report, Merritt et al. (1969) searched the literature and their own material for cases of Huntington's chorea with onset before the age of 21. They found that when the disease began in childhood or adolescence the history was generally on the father's side. In 89 out of 113 cases in which descent could be determined, the father or his relatives were affected. The sex ratio among the affected young people was essentially 1:1. In discussing their results Merritt et al. suggest, referring to Bell (1934), that female choreics have fewer children than male choreics; hence, in a representative sample of choreics, including juvenile cases, we should find that paternal predominates over maternal descent. However, in Bell's work roughly the same percentages of male and female choreics failed to marry, and among those who did marry males and females had approximately the same average number of children. More recent work (Reed and Neel, 1959) finds that fertility is sharply reduced in male but not in female choreics. Reed and Neel report that the average male choreic has 1 8 and the average female choreic 2-8 children, the principal source of the difference being a larger proportion of male choreics who do not marry. It appears, therefore, that in the adult form of the disease, which accounts for all but 1 or 2% of the cases (Myrianthopoulos, 1966), females have more children and maternal descent predominates. Hence, the predominance of paternal descent in the juvenile cases is an exception to the rule in Huntington's chorea. As an alternative explanation, Merritt et al. (1969) suggest that 'affected male parents may induce a shift to a younger age of onset in their offspring', i.e. paternal cases in general have an earlier onset than maternal cases. The purpose of this study is to examine this alternative explanation for the predominance of paternal descent in the juvenile form of Huntington's chorea.