Principles of Genetics and Breeding

2.1.6 Examples for dihybrid inheritance in fishes

When two or more genes are inherited independently (i.e., they are not linked), each phenotype is also inherited independently.

• For e.g. gold body coloration in the guppy is controlled by a simple autosomal recessive gene g. The dominant G allele produces grey guppies. Curvature of the spine is also controlled by a unique autosomal recessive gene cu. The dominant Cu allele produces normal spines.
• The two genes are inherited independently (law of independent assortment). The 9:3:3:1 phenotypic ratio is always produced when two heterozygotes mate and the two phenotypes are controlled by single autosomal genes with complete dominant gene action.
• In minnows the colour and finnage traits were independently controlled by two distinct loci, and within each locus the novel alleles, for pink colouration or long fins, were recessive to those producing the normal phenotype (grey colour and normal fin). Cross of a two heterozygous grey (Gg Nn x Gg Nn) normal results in the production of grey normal-9/16, grey long fin- 3/16, pink normal- 3/16 and pink long fin-1/16.