Principles of Genetics and Breeding

3.1.7 Multiple Alleles

A type of inheritance unknown to Mendel is that called multiple allelism .

• Some genes occur in more than two allelic forms (in contrast to alternative forms). In such cases the various allelic forms are collectively referred to as multiple alleles.
• A set of multiple alleles may contain three, four and even upto twenty or more members and all the members essentially occupy the same locus in homologous chromosomes.
• It therefore follows that regardless of their total number, only two members of a set occur in a diploid cell, and only one in a gamete (haploid cell).
• All the members of a set of multiples alleles are infact the mutant forms of the same gene. Since mutations keep occurring the membership of a set is subject to increase.
• In a set of multiple alleles, one member is always dominant to all the others and one member will be always recessive to all the others.
• The presence of multiple alleles is also reported in fishes.

1. The B gene which controls melanin formation in the Medaka’s melanophores, is an example of an autosomal gene with three alleles. The B allele is dominant over the B’ and b alleles and the B’ allele is dominant over the b allele; the b allele is recessive to the other two.

   Genotype	Phenotype
   BB BB Bb	Full melanin production
   B’ B’ B’b	Variegated pigmentation
   bb	No or minimal melanin pigmentation

2. In platy fish there are nine alleles at the P locus, P⁺, P^M, P^MC, P^r, P^CO, P^C, P^Cc, P^O, P^D

P⁺ - unpotted phenotype which is recessive to all other alleles.

• The remaining alleles are co-dominant. They produce both phenotypes.
• The alleles at the P locus can in theory produce 37 phenotypes but a number of patterns overlap which reduces the number of visible phenotypes to 27.

P^M – Moon, P^MC- -Moon complete, P^r – twin spot, P^CO - comet, P^C- crescent, P^Cc- complete crescent- P^O - one spot, P^D - dot.