Factors which contribute to cytogenetic frequency of expression in families of fragile X females

Abstract

Frequency of expression in mentally retarded (MR) fragile X (fra[X]) males has been shown to be affected by genetic factors and laboratory procedures. Among MR females, factors that contribute to cytogenetic expression have been more problematic in part due to lower frequency of expression and X-inactivation. We examined fra(X) sisters to determine whether genetic factors affecting frequency of expression in sisters were comparable to those in brothers. Evaluations obtained from 2 data sets for a total of 66 families in which at least 2 affected brothers and/or 2 affected sisters were found cytogenetically positive were selected. Of 166 subjects chosen, 118 were male and 48 were female. Sibs were evaluated using an analysis of covariance (ANCOVA) with 2 factors (family, sex) adjusted for the covariate, age. Results indicated that interactive effects (family-by-sex) and main effects (family, sex, age) were significant but the interactive effect of age-by-sex was not. Consequently, one-way analyses of variance (ANOVA) of the effect of family were calculated separately for brothers and sisters. Correlation coefficients between family and cytogenetic frequency was r = .84 for brothers and r = .79 for sisters. Analysis of these coefficients indicated that they were not significantly different from one another (Z = 0.85, p greater than .40). While other factors may affect cytogenetic expression, whatever familial factors control frequency of expression among brothers appear to affect frequency of expression among sisters as well.