Incomplete Dominance

2. Incomplete Dominance Definition
Incomplete dominance is a form of intermediate inheritance in which one allele for a particular
trait is not expressed completely over its paired allele.”
What is Incomplete Dominance?
Incomplete dominance is a form of Gene interaction in which both alleles of a gene at a locus are partially
expressed, often resulting in an intermediate or different phenotype. It is also known as partial dominance.
For eg., in roses, the allele for red colour is dominant over the allele for white colour. But, the heterozygous
flowers with both the alleles are pink in colour.
Mechanism of Incomplete Dominance
Incomplete dominance occurs because neither of the two alleles is completely dominant over the other. This
results in a phenotype that is a combination of both.
Gregor Mendel conducted experiments on pea plants. He studied on seven characters with contrasting traits
and all of them showed a similar pattern of inheritance. Based on this, he generalized the law of inheritance.

3. Later, researchers repeated Mendel’s experiment on other plants. Shockingly, they noted that the F1
Generation showed variation from the usual pattern of inheritance.
The monohybrid cross resulted in F1 Progeny which didn’t show any resemblance to either of the parents, but
an intermediate progeny.
Let’s understand the incomplete dominance with the example of Snapdragon flower (Antirrhinum sp).

4. Monohybrid cross was done between the red and white coloured flowers of Snapdragon plant. Consider, pure
breed of the red flower has RR pair of alleles and that for the white flower is rr.
Firstly, true-breeding red (RR) and white (rr) coloured flowers of snapdragon were crossed. The F1 generation
produced a pink coloured flower with Rr pair of alleles.
Then the F1 progeny was self-pollinated. This resulted in red (RR), pink (Rr) and white (rr) flowers in the ratio of
1:2:1.
Recollect that the genotype ratio of F2 generation in the monohybrid cross by Mendel also gave the same ratio
of 1:2:1. However, the phenotype ratio has changed from 3:1 to 1:2:1. The reason for this variation is the
incomplete dominance of the allele R over the allele r. This led to the blending of colour in flowers.