Multi-locus population genetics - What is linkage disequilibrium?
Linkage disequilibrium is the condition in which the haplotype frequencies in a population deviate from the values they would have if the genes at each locus were combined at random.
The butterfly Papilio memnon provides a good example of linkage disequilibrium. Papilio memnon (pictured opposite) is a swallowtail butterfly living in the Malay Archipelago and Indonesia. The females come in a variety of morphs. The most common morphs mimic natural models which are unpalatable to birds.
Unlike the camouflage polymorphism in the peppered moth, each of the different morphs is thought to be controlled by a multi-locus genotype.
The loci in question are so tightly linked that rare recombinants practically never arise - this explains why the different multi-locus genotypes appear, when crossed, to segregate like single-locus genotypes. A set of genes so tightly linked that they behave like a single locus has been termed a supergene. However, a sufficiently large number of crosses should be able to break one of the supergenes into different combinations.
In the P. memnon there appear to be at least five loci in the mimicry supergene: T, W, F, E, and B. They control respectively presence or absence of tail, hindwing patterns, forewing pattern, epaulette color, and body color. The rare anura morph, for instance, is thought to be a recombinant between the T locus and the other four.
An animation illustrates the research by biologists Clarke and Sheppard which suggested that the genes controlling wing color in P. memnon is due to selection for mimicry.