Mutation-selection balance with multiple alleles.

Human genetic disorders provide an extraordinary richness of data on the diversity of defective alleles. Well over 100 defective alleles for each of several human genetic disorders have been identified, including breast cancer (BRCA1), cystic fibrosis (CFTR), muscular dystrophy (DZM), and phenylketonuria (PAH). These observations raise the classical question of balance between the action of mutation generating new defective alleles and selection removing those alleles from the population. The problem of multiple-allele, mutation-selection balance was considered by Crow and Kimura, who obtained some approximate results showing that the level of dominance and degrees of interallelic complementation are important in determining the equilibrium allele frequencies. Here those deterministic results are reviewed and extended, showing that there are conditions yielding surprisingly high equilibrium frequencies of defective alleles. Just as the equilibrium mutation load is independent of the level of dominance, it is also independent of the number of defective alleles.