Codominance

A red flower RR with two dominant red alleles is crossed with a true breeding (homozygous) white flower rr having two recessive alleles corresponding to the red color. All F1 offspring are heterozygous Rr, displaying red and white codominant phenotypes.

Self-pollination of the codominant F1 plants produces offspring with phenotype 1 red: 2 red/white: 1 white. Thus, alleles are still inherited according to Mendel’s laws, even when they show codominance.

Codominance Punnett Square

To understand the mechanism of codominance and verify if it follows Mendel’s laws of inheritance, the botanists use Punnett square as shown below:

The result thus shows codominance in 100% of F1 offspring and 50% of F2 offspring.

MN Blood Grouping in Humans

It is the most common example of codominance in MN blood groups of humans. A person’s MN blood type is determined by two alleles, M and N, of a particular gene. An L^M allele has an M marker on the surface of the red blood cells (RBCs), while an L^N allele has an N marker.

Individuals homozygous for the M allele have M as the RBC antigen. Similarly, organisms homozygous for the N allele have N as the RBC antigen. Heterozygotes (L^M L^N) have both types of markers expressed equally on the surface of the RBCs.

ABO Blood Grouping in Humans

Persons with type AB blood have one allele for A and the other for B that are dominant. In contrast, the O allele is recessive, and thus its expression is masked by the expression of the other alleles.

The altered hemoglobin that causes sickle-celled anemia is inherited as a codominant trait. Heterozygous individuals express normal and sickle hemoglobin, so they have a mixture of normal and sickle red blood cells.

Codominance in Other Animals

• Speckled chickens have alleles for both black and white features.
• Roan cattle have alleles for both red hair and white hair.

Codominance in Plants

• Rhododendrons express red and white color flower petals simultaneously.

Codominance and incomplete dominance are two genetic inheritance patterns found in living beings where one allele is not completely dominant over the other allele.

What is the Difference between Codominance and Incomplete Dominance

In codominance, both parental phenotypes are expressed together in the offspring. In contrast, in incomplete dominance, the two parental phenotypes blend to form a new phenotype for their offspring. Some other common differences are:

Basis	Codominance	Incomplete Dominance
1. Type of Dominance	Neither of the two alleles is dominant	One allele is completely dominant over the other
2. Phenotype	Both the allele colors blend equally in the offspring	Both allele colors blend, but one of them is expressed
3. Hybrid Phenotype	A hybrid does not give rise to a new phenotype	A hybrid always gives rise to a new phenotype
4. Examples	MN blood grouping in humansABO blood grouping in humansRoan nature in cattle	Pink flowers snapdragon, Antirrhinum majus, formed when a homozygous red flower is crossed with a homozygous white flower.The inheritance pattern of height, weight, and eye color in animals, especially humans
5. Qualitative/Quantitative Approach	It is a qualitative approach to the gene expression	It is a quantitative approach of both of the alleles of a gene