Codominance

Key definition

Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype.

With the previous examples discussed, the involved alleles are either dominant or recessive.
- Remember that the recessive allele is not expressed in the phenotype of individuals with the heterozygous genotype.
Sometimes, both alleles are expressed and neither is dominant. This means the phenotype would be a mixture of the effects of each allele. This is called codominance.
- The alleles are called codominant alleles.

The four o’clock plant can have red flowers or white flowers. Flower color is determined by one gene.
If you cross homozygous red-flowered plants with homozygous white-flowered plants, the offspring are all pink.
- The two parents produce heterozygous offspring (the $F_{1}$ generation) in which both alleles are expressed to give pink, an intermediate color.
A Punnett square can be used to show these characteristics.

gametes	$C^{R}$	$C^{W}$
$C^{W}$	$C^{R} C^{W}$	$C^{W} C^{W}$
$C^{R}$	$C^{R} C^{R}$	$C^{R} C^{W}$

There are three possible combinations for this plant:
- 25% homozygous $C^{R} C^{R}$
- 50% heterozygous $C^{R} C^{W}$
- 25% homozygous $C^{W} C^{W}$
The phenotypes are:
- 25% red
- 50% pink
- 25% white
These ratios can be given as $1 : 2 : 1$ .
Notice also that each allele is represented as a CAPITAL LETTER.
- a red flower would be represented as $C^{R}$
- a white flower would be represented as $C^{W}$ .
- This shows that neither allele is recessive and that they both exert an equal effect on the phenotype.

With the previous examples discussed, it only shows two alternative alleles for a gene. There are examples where a gene may have three or more alleles.

The human ABO blood group system is determined by one gene, $I$ , with three different alleles:
- $I^{A}$
- $I^{B}$
- $I^{O}$
The alleles $I^{A}$ and $I^{B}$ are codominant and code for slightly different molecules on the surface of red blood cells.
The allele $I^{O}$ is recessive to both $I^{A}$ and $I^{B}$ and does not code for one of these molecules.
The four different blood groups are:
- A
- B
- AB
- O

gametes	$I^{O}$
$I^{B}$	$I^{B} I^{O}$
$I^{A}$	$I^{A} I^{O}$

This is an example of two parents that have blood groups AB and O:
- The children will have blood groups A or B. None of them will have the same blood group as their parents.

gametes	$I^{B}$	$I^{O}$
$I^{O}$	$I^{B} I^{O}$	$I^{O} I^{O}$
$I^{A}$	$I^{A} I^{B}$	$I^{A} I^{O}$

This is an example of two heterozygous parents that have blood groups A and B.
- Their children can have any of the four blood groups: A, B, AB, and O.