Determinants of ridge counts in MZ twin kinships

The inheritance of total ridge count (TRC) was studied in 967 individuals from the families of 111 pairs of MZ twins. The sample included data on 47 male half-sibships with 227 offspring and 64 female half-sibships with 306 offspring. The males in this sample had a mean ridge count of 135 +/- 2 and the females a mean ridge count of 124 +/- 2. The distribution of scores for females showed evidence for significant skewness. For this reason, prior to the analysis, the data were corrected for sex and adjusted to normality using a power transformation. Nested analyses of variance were performed on the ridge counts from male and female half-sibships separately to derive estimates of among, between, and within-variance components. These estimates were then used in a nonlinear least squares program to estimate genetic and environmental parameters and to determine the goodness of fit of various models. A model which included additive genetic and dominance effects could not be rejected (P = 0.67) but did not fit the data as well as a simple additive genetic-random environmental model (P = 0.81). The addition of maternal effects to the later model also provided a satisfactory fit (P = 0.68). However, there were no improvement in the goodness of fit over the simple model, and estimated magnitude of the maternal effect was not significantly different from zero.