A cluster of tightly linked genes that affect the same trait and are inherited apparently as a single unit. For example, supergenes are known to determine shell colour and patterning in terrestrial snails, and wing shape and colouring essential to mimicry in certain butterflies. This tight linkage is advantageous since it ensures that the appropriate combination of alleles for the character concerned is virtually always transmitted intact to the animal’s offspring. Any rare recombinants will be selected against. A supergene can arise through inversion of part of a chromosome; crossing over within the inversion produces unbalanced recombinant chromatids, and hence zygotes containing these are nonviable. Thus, with respect to the inversion, only nonrecombinant zygotes are generally produced, giving a very low apparent rate of recombination between the loci in this region.