incomplete dominance
Picture mixing red and white paint and getting pink. In incomplete dominance, a heterozygote shows a phenotype halfway between the two homozygotes — neither allele fully overrides the other, and the result is a blend rather than a clear-cut winner.
Mechanistically, this usually happens because one allele makes a working product and the other makes little or none, so a heterozygote with only one good copy produces an intermediate amount. With a pigment, half the normal dose can yield a paler, in-between color rather than the full shade.
It helps to contrast the three patterns. In full dominance, one allele's phenotype completely hides the other; in codominance, both alleles' distinct phenotypes appear together; in incomplete dominance, the two merge into a single intermediate. Incomplete dominance also explains why crossing two intermediates can give a clean 1:2:1 ratio of the three visible types in the next generation.
Crossing a red-flowered snapdragon with a white one yields pink offspring; the heterozygote makes about half the pigment, giving an intermediate shade.
Red × white gives pink: a blended, intermediate heterozygote.
Incomplete dominance does not contradict Mendel: the alleles still segregate cleanly. Only the visible phenotype of the heterozygote is intermediate, not the alleles themselves blending.