Incomplete dominance

Incomplete dominance describes a pair of alleles of one gene in which neither fully hides the other. An individual with one copy of each, a heterozygote, therefore looks intermediate between individuals carrying two copies of either allele. The classic example is flower color in snapdragons: crossing a red-flowered plant with a white-flowered one gives offspring that are all pink.

The usual explanation is gene dosage. Often one allele makes a working protein, such as an enzyme for red pigment, while the other makes little or none, so a plant with a single working copy produces only part of the full amount of pigment. The alleles themselves are not blended or changed, so the parental colors return in the next generation: crossing two pink plants gives red, pink and white offspring in a 1:2:1 ratio. Because this matches the genotype ratio, every genotype can be recognized by its appearance.

Incomplete dominance differs from codominance, in which a heterozygote shows both parental traits in full rather than an in-between form. Whether dominance looks complete can depend on the level examined: carriers of Tay-Sachs disease are healthy, yet their cells make only about half the usual amount of the enzyme involved.