Population genetics implications of the premutation hypothesis for the generation of the fragile X mental retardation gene.

The population genetics implications of the premutation hypothesis for the generation of the fragile X mental retardation gene are explored. With some broad assumptions, the consequences of the model are that 50% of mothers of probands carry the premutation; 6.5% of mothers of probands receive the premutation from their mothers, 18.9% from their fathers, and 24.6% as a "new mutation"; the incidence of carriers for the full mutation equals the incidence of affected males, whereas the incidence of carriers for the premutation is 1.35 times the incidence of affected males; assuming mutation rates are equal in eggs and sperm, the mutation rate from normal to premutation alleles is 1.67 X 10(-4); the expected segregation ratio in sibs of probands is 0.44, which corresponds to observed values. In addition, predictions using the premutation hypothesis of the expected segregation ratio in sibs of mothers of probands fits well with the data of Vogel and coworkers.