Multiple Alleles

Every gene considered in a simple Mendelian cross was treated as though it had only two possible forms, a dominant allele and a recessive allele, each occupying a fixed locus on a pair of homologous chromosomes. In reality, many loci exist in a population as an entire series of three, four, or even more distinct allelic forms, called multiple alleles, all of which can occupy that same single locus. The key point is that this multiplicity exists only at the level of the whole population: any single diploid individual, having only two homologous chromosomes at that locus, ca …