Question

In a Mendelian monohybrid cross, the F${}_2$ generation shows identical genotypic and phenotypic ratios. What does it tell us about the nature of alleles involved? Justify your answer.

Answer 1

Monohybrid cross

Monohybrid cross is a cross between parents having different alleles at a single gene locus for a particular trait. In a Mendelian monohybrid cross, the F${}_2$ generation shows non-identical genotypic (1:2:1) and phenotypic (3:1) ratios.

Incomplete dominance

The alleles exhibiting incomplete dominance lead to the blending of contrasting traits in heterozygous condition. A cross between true-breeding varieties produces heterozygous progeny whose phenotype is different from that of the parents.

Example of incomplete dominance

Example of four o'clock plant (Mirabilis jalapa):

A cross between a pure red flowered plant (RR) and pure white flowered plant (rr) yields hybrid pink flowered plant (Rr) due to incomplete dominance.

Here, the dominant allele R is not able to completely mask the effect of the recessive allele r. So, they show identical genotypic and phenotypic ratios in F${}_2$ generation, that is 1:2:1.



F${}_1$ generation of incomplete dominance

Cross between red (RR) and white (rr) flowered plants produced all pink progeny (Rr) in F${}_1$ generation.

	R	r
R	Rr Pink	Rr Pink
R	Rr Pink	Rr Pink

F${}_2$ generation of incomplete dominance

Cross between two hybrid pink (Rr) flower plants produces red (RR), pink (Rr) and white flower (rr) plants in F${}_2$ generation.

	R	r
R	RR Red	Rr Pink
r	Rr Pink	rr White

Phenotype and genotype in F${}_2$ generation

Phenotype and genotype in F${}_2$ generation are as follows:

Characteristics	Pure red flower (RR)	Pink flower (Rr)	Pure white flower (rr)
Genotype	1	2	1
Phenotype	1	2	1