Intrafamilial correlation in Friedreich's ataxia.

The distribution of age of onset and age of losing the ability to walk independently in patients with Friedreich's ataxia from 93 sibships are presented. Intrafamilial correlation was investigated using three methods: the Pearson product-moment correlation coefficient and to estimates of correlation based on analysis of variance, one a maximum likelihood method and the other using a weighted mean family size. All three resulted in correlation coefficients (r) of around 0.5 for both disease parameters. The latter two methods yielded similar values just greater than 0.5 and it is considered that these are more reliable estimates of intraclass correlation as there were a variable number of family members in the sibships studied. Although the examination of intraclass correlation in Friedreich's ataxia was not highly suggestive of genetic heterogeneity, the observed proportion of first cousin marriages amongst the parents of the sibships studied was about twice the expected proportion. Overall, the data would be compatible with genetic heterogeneity if one genotype is more prevalent than others. An alternative hypothesis is that there are different modifying genes operating within different families in which Friedreich's ataxia occurs.