Intratetrad mating and the evolution of linkage relationships.

Mating among the immediate products of meiosis (intratetrad mating) is a common feature of many organisms with parthenogenesis or with mating-type determination in the haploid phase. Using a three-locus deterministic model we show that intratetrad mating, unlike other systems of mating, allows sheltering of deleterious recessive alleles even if there is only partial linkage between a mating locus and a load locus. Moreover, modifiers that reduce recombination between the load and mating-type locus will spread to fixation, even when there is no linkage disequilibrium between these loci in the population as a whole. This seeming contradiction to classical expectation is because partial linkage generates linkage disequilibrium among segregating loci within a tetrad, which then acts as the "mating unit."