Incomplete Dominance

Incomplete dominance occurs when neither allele is fully dominant over the other, so the heterozygote displays an intermediate phenotype that blends the two parental traits. In snapdragon (Antirrhinum majus), crossing a homozygous red (RR) with a homozygous white (rr) plant gives Rr heterozygotes that appear pink, an intermediate between red and white. This happens because a single dose of the pigment-producing allele makes only enough pigment for a diluted colour. Codominance is incorrect because in codominance both alleles are fully and simultaneously expressed, producing patches or both traits together rather than a blend, as in human AB blood group. Complete dominance is wrong since that would yield a fully red F1, masking the recessive allele entirely. Epistasis involves one gene masking the expression of another gene at a different locus, which is not the case here as only one gene controls flower colour. As detailed in NCERT Class 12, Chapter 5 (Principles of Inheritance and Variation), the F2 of such a cross shows a 1:2:1 ratio in which genotype and phenotype ratios coincide. Checking consistency, the intermediate pink phenotype reappearing as a distinct heterozygous class confirms incomplete dominance rather than blending inheritance, since red and white reappear pure in F2.