incomplete dominance

Meaning: In complete dominance, the heterozygote looks like the dominant homozygote. In incomplete dominance, the heterozygote has a blended, intermediate phenotype. A common explanation is that one functional allele makes too little product to give the full dominant phenotype, so the result falls between the two homozygotes. The alleles themselves do not blend or change; they separate normally in the next generation.

On the MCAT: A passage gives a cross, such as red (RR) × white (rr) snapdragons, and asks for offspring ratios. All F₁ are Rr and pink. Crossing two pink plants gives 1 red : 2 pink : 1 white, so the genotypic and phenotypic ratios match (1:2:1). This differs from the 3:1 phenotypic ratio of complete dominance. You may also be asked to infer the mode of inheritance from a ratio: if the F₂ shows three phenotypes in a 1:2:1 ratio, think incomplete dominance. Alleles are often written with the same letter in capitals, such as CR and CW, because neither is dominant.

Common confusion: Incomplete dominance is not codominance. In codominance, both alleles are fully and separately expressed in the heterozygote, as in AB blood type or a red-and-white spotted flower. In incomplete dominance, the heterozygote shows a single intermediate trait, such as uniformly pink. It also is not Mendel's law of segregation being broken: the parental phenotypes reappear in the F₂, which shows the alleles stayed distinct.