Question

Three Mark Questions :
The $F_2$ progeny of a monohybrid cross showed phenotypic and genotypic ratios as 1 : 2 : 1 unlike that of Mendel's monohybrid $F_2$ ratio. With the help of a suitable example, work out a cross and explain how it is possible.

Answer 1

Such a condition in which both the genotypic and phenotypic ratios are same is an example of incomplete dominance. In snapdragon plants (Antirrhinum), a cross between a red flowered (RR) plant and a white flowered plant (rr) results in all F1 generation plants being pink flowered (Rr), an intermediate phenotype. When a cross is carried out between two pink flowered plants, it results in an F2 generation where red, pink and white flowers are seen in the ratio of 1:2:1. They have a genotypic ratio of RR (Red): Rr (pink): rr(white) that is equal to 1:2:1. So, both the phenotypic and the genotypic ratios are same.