Incomplete Dominance
குறை ஓங்குதன்மை
Do not confuse Incomplete Dominance with Codominance. In Mendelian genetics, the dominant allele masks the recessive one completely. Here, the heterozygote phenotype is a blend. It is a direct violation of the Law of Dominance.
The idea
Incomplete dominance occurs when the F1 generation shows a phenotype intermediate between the two homozygous parents. In Antirrhinum majus (snapdragon), a cross between red (RR) and white (rr) flowers produces pink (Rr) F1 progeny. The genotypic ratio remains 1:2:1, identical to the phenotypic ratio. This confirms that the alleles do not blend, but the expression of the dominant allele is incomplete.
Why it matters
It exists because one allele is not completely dominant over the other, resulting in a blended expression of the gene product.
NEET lens
NEET asks for the phenotypic and genotypic ratios specifically. Remember: both are 1:2:1. Do not mistake the pink color for a new allele; it is the result of the Rr genotype. Expect questions comparing this with Codominance using blood grouping as a distractor.
Clinical significance
Incomplete dominance is observed in human Familial Hypercholesterolemia. Individuals with one mutant allele show elevated cholesterol levels, while those with two mutant alleles suffer from severe, life-threatening cardiovascular disease. It demonstrates how gene dosage affects clinical severity.
Introduction
Incomplete dominance (partial dominance) is a pattern of inheritance where neither allele in a heterozygote is fully dominant, producing an intermediate phenotype that appears to be a 'blend' of the two parental phenotypes.
Core concept
Examples: Flower colour in Mirabilis jalapa (four o'clock plant): RR (red) × WW (white) → F1: RW (pink) — intermediate. F1 × F1: RW × RW → 1 RR (red) : 2 RW (pink) : 1 WW (white). Phenotypic and genotypic ratios BOTH = 1:2:1 (different from Mendelian 3:1 phenotypic). Snapdragon (Antirrhinum): red × white → pink F1. Andalusian fowl: black × splashed white → blue Andalusian. Mechanism: RR → two doses of pigment → red. WW → no pigment → white. RW → one dose → half pigment → pink (pigment 'diluted'). Neither allele is dominant — but in Mendelian dominance, TT and Tt both tall because one T is enough for full height. In incomplete dominance, one copy of allele is insufficient for full phenotype.
Go deeper
Why incomplete dominance at molecular level? If protein product is rate-limiting (phenotype requires a threshold amount of protein), then: TT → enough protein → full phenotype. Tt → half protein → intermediate phenotype. tt → no protein → absent phenotype. The gene product (e.g., enzyme producing pigment) has a gene-dosage effect. In contrast, in complete dominance: one copy of T is sufficient for full phenotype → Tt = TT.
From Class 10
In incomplete dominance, neither allele is fully dominant. The heterozygote shows a blended, intermediate phenotype — like mixing red and white paint to get pink.