Definition
Incomplete dominance is a pattern of inheritance in which neither of two alleles fully overpowers the other. When an organism is heterozygous, it shows a look halfway between the two parents. It is a classic case where Mendel's principle of dominance does not apply in its simple form.
Red Meets White and Makes Pink
Mix red paint and white paint on a palette and you get a soft pink. Heredity sometimes works like that, too. An offspring can end up with a trait halfway between its parents.
Under Mendel's basic rules, one of the two alleles completely covers the other. Cross a pure-breeding yellow pea with a pure-breeding green pea, for example, and every offspring is yellow, the dominant color. But nature is full of cases that aren't so black and white.
A classic example is flower color in the four o'clock plant or the snapdragon. Cross a pure red flower with a pure white flower and, surprisingly, you get a pink flower, a color neither parent had. Neither allele could fully overpower the other, so a middle form appeared.
Did the Genes Melt Together Like Paint?
Looking at a pink flower, it's easy to think what scientists of the past thought: that the parents' blood, or their genes, blended together like batter and can never be separated again. This is called the blending theory of inheritance. But unlike paint, genes never melt into each other.
Here is an interesting way to test it. Cross two pink flowers with each other and look at the next generation. If the genes had truly blended, every offspring should be pink.
In reality, the offspring come out in a ratio of 1 red, 2 pink, and 1 white. The pure red and pure white that seemed to have vanished come back in full in the next generation. Genes don't blend and disappear. They stay intact, like separate little grains, which means Mendel's law of segregation is still at work.
A Closer Look
Look inside the cell at the molecular level and the secret of incomplete dominance turns out to be simpler than you might expect. A common explanation for flowers like the snapdragon is that a pigment gene is really the blueprint for an enzyme protein that builds red pigment.
A pure red flower has two red genes, so it makes plenty of the enzyme, a full 100%, and shows a deep red. A pure white flower has only white genes, so it makes none of the pigment-building enzyme and stays white.
What about a hybrid flower with one red gene and one white gene? It has only one working blueprint, so it makes roughly half the amount of the pigment-building enzyme. With less red pigment made, our eyes see a pale pink. In other words, a difference in how much one gene produces (its expression level) creates the in-between look.
Common misconceptions
In incomplete dominance, the genes themselves blend together and turn into a new, in-between gene.
The genes don't blend. They stay intact. When pink flowers are crossed again, the original red and white genes separate, and red and white flowers show up again in the next generation.
Where you meet it
Crossing a red flower with a white flower in the four o'clock plant or snapdragon produces pink flowers.
Crossing a black-feathered Andalusian chicken with a white-feathered chicken produces chicks with bluish-gray feathers.
In one sentence
Incomplete dominance is when neither allele fully dominates, so an in-between look appears, yet the genes themselves never blend and stay intact.