Notes on the population genetics of fragile X syndrome

Our analysis of fragile X-inactivation in normal and mentally retarded heterozygotes led us to conclude that a fraction of female carriers of the imprinted (fully mutated) allele is phenotypically normal as a consequence of X-inactivation. Taking this into account, we derived equilibrium equations for the fragile X [fra(X)] genotype frequencies. We also showed that small variations in the value of s (selection coefficient of affected heterozygotes) and r (imprinting rate during oogenesis) affect genotype ratios significantly.