In this article we will discuss about the examples of incomplete dominance in genetics.
Mendel’s law of dominance states that out of a pair of allelomorphic, contrasting or alternative characters one is dominant and the other recessive. For example, when Mendel crossed a true breeding red flowered plant with a true breeding white flowered one, the offspring produced was found similar to the red flowered parent. The whiteness appeared to be suppressed and the redness-to dominate.
This fact was found to be true between all the seven pairs of characters studied by Mendel. However, there are few characters which are exceptions to this rule and show partial or incomplete dominance. The dominance is here absent and the hybrid individuals resemble of the parents but more or less intermediate between the two.
For example, the F1 progeny from a cross of red and white flowered Mirabilis jalapa (four o’ clock plant) bear pink flowers. This would then give rise to red, pink, and white flowered plants in 1: 2: 1 ratio in F2 generation.
Similarly in Snapdragon (Antirrhinum majus) plants with broad leaves and plants with narrow leaves give rise to plants with intermediate leaves in F1 generation and 1 broad, 2 intermediate, and 1 narrow leaved plants in F2 generation.
Example of another incomplete dominance is seen in animal kingdom. In Andalusian fowls or chickens, if we cross a black feathered fowl with white feathered one, in F1 all the fowls present a blue feathered appearance. When self-fertilized, the blue birds produce black, blue and white in the ratio of 1: 2: 1.
Similarly, in short horned cattle, if we cross a red bull with a white cow, the F1 offspring shows an intermediate colour and are called roan or variegated. When the roans are crossed along themselves, the F2 offsprings consist of red, roan and white in the ratio of 1: 2: 1. The hair of roan animal is either pure red or pure white and a mixing of two types produces an intermediate shade.
All the examples of incomplete dominance can be explained on the basis of Mendelian segregation. In fact, in case of complete dominance, the recessive factors fail to show its effect in the presence of dominant factor. However, in case of incomplete dominance, both alleles have almost equal effect on the phenotype. The result is intermediate character of the hybrid. This type of inheritance is also called as blending inheritance.