Incomplete Dominance and Codominance

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In incomplete dominance neither allele is fully dominant, and heterozygotes show an intermediate blend: red × white snapdragons give pink offspring. In codominance both alleles are expressed fully and separately: the AB blood group has both A and B antigens, and roan cattle have both red and white hairs.

These patterns show that Mendel's simple dominant/recessive rule is not the only possibility. Ratios in the offspring reveal which pattern applies.

In roan cattle, a red parent and a white parent can produce offspring with both red and white hairs mixed, a case of codominance, since both alleles show up separately. Human blood group AB is another: the person makes both A and B antigens. In incomplete dominance, the pink snapdragon is a smooth blend, and crossing two pink plants gives red, pink and white in a 1:2:1 ratio.

These patterns show that Mendel's simple dominant and recessive model does not cover everything. In sickle-cell trait, carriers make both normal and sickle haemoglobin, and show mild effects. Students mix up the two terms: codominance shows both traits separately, such as spots of two colours, whereas incomplete dominance shows a mixture, like pink paint. Also note the blending does not permanently mix the alleles, as red and white reappear in later generations.