A red flowered four o'clock plant crossed with a white flowered one gives pink flowered offspring. This is an example of

Explanation

In incomplete dominance the heterozygote shows a blend, because one dose of the functional allele makes too little pigment for full colour. It is distinguished from codominance, in which both alleles are fully and separately expressed and the heterozygote shows both, as with the roan coat of cattle. The F2 phenotypic ratio here becomes 1:2:1, matching the genotypic ratio.