Incomplete Dominance & Mendel's Experiment

Going back to ancient times, Gregor Mendel and his experiment on pea plants. Based on the Mendel’s work on pea plants, he proposed three laws, commonly known as Mendel’s laws of inheritance. However, the Mendelian experiments were not universal for every plant. Some plants showed variations in their phenotype.

Concept of Dominance

In genetics, Dominance  is a relationship between alleles of one gene.In order to understand the concept of the dominance of alleles, we need to know more about genes. So far we know that genes are a hereditary unit in organisms which exist as a pair of alleles in diploid organisms. These pair of alleles may or may not be similar. That is, a heterozygous gene has two dissimilar pairs of alleles while homozygous have identical ones. Thus, heterozygous alleles carry different information on traits. When we say one trait is dominant over the other, there can be two reasons: either it is non-functional or is less active than the normal allele.

Let’s take a glance at incomplete dominance with an example.

Incomplete Dominance

Incomplete dominance is a form of Gene interaction, in which, a heterozygous condition, both alleles of a gene at a locus are partially expressed, often resulting in an intermediate or different phenotype. It is also called partial dominance.

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.

Later, researchers repeated Mendel’s experiment on other plants. Shockingly, they noted that 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).

Monohybrid cross was done between the red and white colored 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) colored flowers of snapdragon were crossed. The F1­ generation produced a pink colored 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 color in flowers.

Incomplete dominance

Codominance

Codominance is closely related to incomplete dominance. In codominance, both alleles can be seen in the phenotype at the same time. Instead of being uniformly pink, a flower with red and white alleles that show codominance will have patches of red and patches of white. As with incomplete dominance, the F2 generation from heterozygous plants will have a ratio of 1:2:1 of red, spotted, and white flowers. Codominance is also shown in humans with AB blood type; the alleles for blood types A and B are both expressed.

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Practise This Question

If both the alleles of a heterozygote show expression, the phenomenon is called ___.