The F₂ progeny of a monohybrid cross showed phenotypic and genotypic ratio as 1 : 2 : 1, unlike that of Mendel’s monohybrid F2 ratio. With the help of a suitable example, work out a cross and explain how it is possible.

A Monohybrid cross is the hybrid of two individuals with homozygous genotypes which results in the opposite phenotype for a certain genetic trait.

When red (RR) and white (rr) snapdragon flowers were crossed, the F₁ generation flowers were all pink (Rr) in colour. When the F₁ parents where self crossed, the F₂ progeny showed new trait of pink coloured flowers along with parental traits namely red and white. This is due to blending of both the dominant and recessive traits, wherein intermediate colours were observed, that is pink in both F₁ and F₂ generations. This is called incomplete dominance.

Now, let’s see the possible genetic combinations in the Punnett square, by combining the parent alleles

F1 generation: all flowers are pink (Rr)

F2 generation: in this, phenotypic ratio is 1(red):2 (pink):1 (white)

genotypic ratio is 1(RR):2(Rr):1(rr)

hence, the phenotypic and genotypic ratios are same.

But, if you compare the same with Mendels trait of pea flowers with purple and white colour, it is as shown below:

F1 generation: all flowers are purple(Pp)

F2 generation: here, phenotypic ratio is 3(purple):1(white) genotypic ratio is 1((PP):2(Pp):1(pp)