A two-locus model for the inheritance of a familial disease.

A model for the inheritance of a disease involving genes at just two linked loci is presented and discussed. One of the two pairs of opposite double homozygotes is assumed to lead to the disease and death. The other genotypes are assumed to be less fit than the double heterozygote. The cause of this reduced fitness may or may not be due to the disease. Conditions for the existence of a stable equilibrium are presented. The model has a sound biological basis and could be used to explain a wide range of disease frequencies and patterns of inheritance. Mutation is not required to explain why the disease persists in the population. Recurrence risks for sibs and twin concordance rates are derived and the consequences of the relaxation of selection against those with the disease are predicted. The effects of a selection model of the kind described on the amount of linkage disequilibrium in the population and on the estimation of the frequency of recombination are discussed.