Difference between Incomplete Dominance and Codominance

Incomplete dominance and codominance are two important phenomena observed in genetics. Both of these phenomena are occasionally confused so it is necessary to find out the difference between incomplete dominance and codominance.

Incomplete dominance

It is a phenomenon in which the phenotype of heterozygote (Aa) is intermediate between phenotypes of two homozygotes (AA and aa). This phenotype is observed when two true breeds for a particular trait are crossed with each, both alleles are not fully expressed in the next generation and an intermediate phenotype is observed.

This concept can be understood by taking the example of the mixing of colors to make a new color. For example, you have white and red colors and you get pink color after mixing both. In this example, white and red colors are the phenotype of true breeds in the parental generation. While pink (an intermediate color) is the phenotype of the first filial generation due to incomplete dominance.

Example of incomplete dominance

Incomplete dominance was the first study by Carl Correns in 1899 in a 4 O’ clock flowering plant. He observed pink flowers when a cross between red and white flowering plants was performed. The pink phenotype in the daughter generation is a clear example of incomplete dominance.

Similarly, wavy hair is an example of incomplete dominance. This is observed when curly hair and straight hair parents give rise to a baby with the heterozygous genetic combination.

Video lesson on incomplete dominance

Codominance

It is a phenomenon in which different alleles of a gene express independently in the heterozygote (A1A2) and form their respective products (XY). This phenotype is observed when two true breeds for a particular trait are crossed with each, both alleles are fully expressed in the next generation and products of both alleles are observed as phenotype.

This concept can be understood by taking the example of the mixing of water and oil together. Both keep their own identity and you can distinguish water and oil from a mixer.

Example of codominance

MN blood group system is an example of codominance which was first discovered by Landsteiner and Levine. This is based on the presence of antigens in red blood cells. Three phenotypes M, N, and MN can be observed in the MN blood group. If a man of the M blood group marries a woman of the N blood group, all their children will have the MN blood group. So all the children are showing codominance.

Video lesson on codominance

Difference Between Incomplete Dominance and Codominance

The main difference between Incomplete Dominance and Codominance are listed in the table below.

Incomplete Dominance	Codominance
The phenotype of heterozygote (Aa) is intermediate between phenotypes of two homozygotes (AA and aa).	different alleles of a gene express independently in the heterozygote (A1A2) and form their respective products (XY).
Intermediate of the two alleles	Independent effect
A new phenotype is produced	Both phenotypes of parents are produced
Genotype is represented as Aa	Genotype is represented as A1A2
Example: Pink flowers of 4 O’ clock plant	Example: MN blood group system of human

Diagram of difference between incomplete dominance and codominance

Video lesson on difference between incomplete dominance and codominance

Some questions and answers

1. Define incomplete dominance

A. It is a phenomenon in which the phenotype of heterozygote (Aa) is intermediate between phenotypes of two homozygotes (AA and aa).

2. Define codominance

A. It is a phenomenon in which different alleles of a gene express independently in the heterozygote (A1A2) and form their respective products (XY).

3. State two differences between Incomplete Dominance and Codominance

A. In case of incomplete dominance, the Phenotype of heterozygote (Aa) is intermediate between phenotypes of two homozygotes (AA and aa). A new phenotype is produced. While in the case of codominance, different alleles of a gene express independently in the heterozygote (A1A2) and form their respective products (XY). Both phenotypes of parents are produced.

5. Give one example of incomplete dominance

A. wavy hair is an example of incomplete dominance. This is observed when curly hair and straight hair parents give rise to a baby with the heterozygous genetic combination.

6. Give one example of codominance

A. MN blood group system is an example of codominance. Three phenotypes M, N, and MN can be observed in the MN blood group, while the MN blood group is due to codominance.