Notice in the above example there were only three genotypes in the offspring generation. However, when both loci are considered jointly, the progeny generation is not in joint equilibrium. The parent population was created by selection, by removing all but two genotypes. Thus, selection has caused the joint disequilibrium. Mixing populations of animals with different gene frequencies will also cause joint disequilbrium, and in small populations, chance can affect gametic frequencies and thereby cause joint disequilibrium.
Joint disequilibrium can change genotypic variance.
Joint disequilibrium is also known as linkage disequilibrium and gametic phase disequilibrium . The word 'linkage' usually implies that loci are physically on the same chromosome and within a short distance of each other so that certain pairs of alleles at the two loci are more common in the population than others. However, linkage disequilibrium does not imply physical linkage of loci, but it is a popular term that appears most frequently in the literature. Measuring Disequilibrium Using the previous example, calculate the actual percentages of each gamete that would be produced from the progeny generation, as shown in the following table. The coupling heterozygotes could be occuring with frequency 2 ru , and then the repulsion heterozygotes would be occuring with frequency 2 st .