Modification of monohybrid ratio:
The mono hybrid phenotypic ratio of 3:1 occurs only in process in which the alleles have a relationship of dominance and recessiveness between each other, in the absence of such a typical allelic interrelationship, the 3:1 phenotypic ratio is variously modified as described below:
1) Incomplete dominance:
When alleles of monohybrid have an interrelationship of incoplete dominance or codominance, the typical phenotypic expression gets modified into 1:2:1. This is so because the dominant allele does not mask completly the phenotypic expresion of the recessive allele. This results in the blending of phenotypic expression. The alleles maintian individuality and seggreagate from each other during gametogenesis. The F2 projeny produced exhibit the phenotypic and genotypic ratios of 1:2:1.
When a homozygous red flowered pea plant is crossed with a homozygous white flowered pea plant, the F1 heterozygotes are found to have pink flowers. When F1 hybrids are bred amongst themselves the F2 generation have identical phenotypic and genotypic ratios of 1 red(RR):2 pink(Rr):1white (rr)
Similar results have been reported in snapdragons and four ‘o’ clock plants too. Amongst animals, the incomplete dominance is well explained and Andalusian ducks. In Andalusian ducks, the black color of the feathers has been found to dominate incompletely over the splashed white color of the feathers. When a black color duck is crossed with white feathered one, the F1 hybrids have blue feathers. The blue feathered F1 hetero zygotes produces a F2 progeny in the phenotypic and genotypic ratio of 1:2:1.