Characteristics of Multiple Alleles

A true multiple-allelic series follows a consistent set of rules that distinguish it from an ordinary single pair of alleles. Every allele in the series occupies exactly the same locus on the homologous chromosome, which has an important structural consequence: because they all sit at one position rather than being spread along the chromosome, there is no crossing over possible between the different alleles of the same series, since crossing over requires two distinguishable loci to exchange material between. All the alleles in a given series, however many there are, govern the very same character or trait; they are simply different versions of the instructions for that one trait, not different traits. Typically, the allele found in the wild population (the wild-type allele) behaves as dominant, while alleles that arose more recently as mutations tend to be recessive to the wild type, though a mutant allele can also behave as co-dominant with, or show an intermediate effect relative to, another mutant allele in the series. Finally, whenever two different mutant …