Abstract

The effect of the fragile X allele on ridge breadth, height and testicular volume was examined using robust statistical techniques for the data collected from 8 families from Ibadan, south west Nigeria, afflicted with this disorder. There is the presence of outliers, an estimated 6.5% for testicular volume and 1.3% for ridge breadth and height data respectively. It is shown that fragile X affects ridge breadth, height and testicular volume in a different manner. Fragile X women had a greater mean ridge breadth than normal women; a pattern similar to normal and fragile X men but the differences were not significant. Fragile X men were shorter than normal men, but no significant difference between the mean height of normal and fragile X women was observed. Whereas fragile X girls were shown to grow more quickly and to stop growth earlier than normal girls, normal women were taller than fragile X women. Testicular volume in fragile X boys continue in development long after normal boys have stopped; an observation that could explain the significant difference in means of adult males. An examination of the covariance between relatives classified according to fragile X status showed that for the three traits the influence of fragile X alleles was to reduce the covariance between parents and offspring, the effect of which produces a departure from an additive polygenic model of inheritance.

Key words: Fragile X allele, ridge breadth, testicular volume, single-gene disorder.