Incomplete Dominance and Codominance

Simple dominance hides one allele's effect completely. Incomplete dominance and codominance are the two ways heterozygotes break that pattern. In incomplete dominance, neither allele fully masks the other, so the heterozygote phenotype lands in between the two homozygotes. Cross a red snapdragon (RR) with a white one (rr) and every Rr offspring is pink, not red, not white, because red pigment production is only half dosage. In codominance, both alleles are fully and separately expressed at once, with no blending. The classic case is AB blood type: a person with genotype I^A I^B makes both A and B surface antigens on every red blood cell, full strength, side by side. Pink is a mixture that looks new; AB is two old things shown together.

Both patterns still follow Mendelian segregation underneath. Cross two pink snapdragons (Rr x Rr) and you get a 1:2:1 ratio of genotypes RR, Rr, rr, which shows up directly as phenotypes: 1 red, 2 pink, 1 white. So pink makes up half of the offspring. That 1:2:1 phenotype ratio, instead of the usual 3:1, is the giveaway that a cross involves incomplete dominance rather than simple dominance.

This topic sits right next to multiple alleles and epistasis in the unit, because all three show that one gene can affect phenotype in ways beyond a clean dominant-recessive split. The alleles never merge into a single physical entity; a pink Rr snapdragon still carries one full R allele and one full r allele, which separate again during meiosis. Cross that pink plant with a white one and you can still get pure white offspring, proof the white allele survived intact inside the pink parent.