Theories of quasi-linkage and "affinity": some implications for population structure.

"Quasi-linkage" refers to nonrandom assortment of genes located on different chromosomes. Although this phenomenon has been widely observed in many organisms since the early part of this century, it is barely known. Interest in it was recently rekindled by the report of an association between quasi-linkage and the expression of genes belonging to a group of cancer viruses whose genomes are integrated in mouse chromosomes. This prompted an examination of the question whether "sustained meiotic affinity," which is one of the explanations proposed for quasi-linkage, can influence population structure in a manner unattainable by other known modes of heredity. It is shown for a two-locus two-allele system that equilibrium is attained with the gametic phase disequilibrium D greater than 0, leading to a permanent excess of the preferred genotypes. The possible relationship of these concepts to the inheritance of susceptibility to cancer and other diseases is discussed.