Question: It is sometimes observed that the F progeny has a phenotype that does not resemble either of the two parents and has intermediate phenotype. Explain by taking a suitable example and working out the cross upto F2 5 progeny
The correct answer is –
The phenomenon described in the question is called incomplete dominance or partial dominance, where the heterozygote shows a phenotype that is intermediate between the two homozygous parents. A classic example of incomplete dominance is seen in the flower color of snapdragons.
In snapdragons, there are two alleles for the flower color gene: the red allele (R) and the white allele (W). The red allele is dominant, while the white allele is recessive. Therefore, a homozygous red snapdragon (RR) will have red flowers, while a homozygous white snapdragon (WW) will have white flowers.
If a heterozygous snapdragon (RW) is produced by crossing a homozygous red snapdragon (RR) with a homozygous white snapdragon (WW), the resulting F1 generation will have pink flowers, which is an intermediate phenotype between red and white. This is because both the red and white alleles contribute to the expression of flower color in the heterozygote, resulting in a blending of the two colors.
If we perform a cross between two F1 heterozygous snapdragons (RW x RW), the resulting F2 generation will have a phenotypic ratio of 1 red: 2 pink: 1 white. This is because the genotypic ratio of the F2 generation will be 1 RR: 2 RW: 1 WW. The homozygous red snapdragons (RR) will have red flowers, while the homozygous white snapdragons (WW) will have white flowers. The heterozygous snapdragons (RW) will have pink flowers, which is an intermediate phenotype between the red and white flowers.
Therefore, incomplete dominance results in the production of F1 progeny with an intermediate phenotype that does not resemble either of the two parents. When these F1 progeny are crossed with each other, the F2 progeny exhibit a classic Mendelian segregation of phenotypes and genotypes in a 1:2:1 ratio.
