# 98 Codominance and inheritance of blood group
Sometimes, neither of a pair of alleles is completely dominant or completely recessive. Instead of one of them completely hiding the effect of the other in a heterozygote, they both have an effect on the phenotype. This is called codominance.
The result is that there can be three different phenotypes. When writing the genotypes of codominant alleles, the common convention is to use a capital letter to represent the gene involved, and a small raised letter for each phenotype.
Imagine a kind of flower which has two alleles for flower colour. The allele Cw produces white flowers, while the allele CR produces red ones. If these alleles show codominance, then the genotypes and phenotypes are:
genotype phenotype
Cw Cw white flowers
Cw CR pink flowers
CR CR red flowers
Common misconceptions
When factors are codominant, students often think this will result in different proportions of offspring having the parents’ features. However, codominance results in the appearance of a new characteristic, which is intermediate to the parents features. For example, if the parents are pure-breeding for long fur and short fur, the offspring will all have medium-length fur.
Inheritance of A, B, AB and O blood group - an example of codominance
Imagine a kind of flower which has two alleles for flower colour. The allele Cw produces white flowers, while the allele CR produces red ones. If these alleles show codominance, then the genotypes and phenotypes are:
genotype phenotype
Cw Cw white flowers
Cw CR pink flowers
CR CR red flowers
Common misconceptions
When factors are codominant, students often think this will result in different proportions of offspring having the parents’ features. However, codominance results in the appearance of a new characteristic, which is intermediate to the parents features. For example, if the parents are pure-breeding for long fur and short fur, the offspring will all have medium-length fur.
Inheritance of A, B, AB and O blood group - an example of codominance
- In humans, there are 4 blood types (phenotypes): A, B, AB, and O
- Blood type is controlled by 3 alleles: IA, IB, IO (the base letter = I stands for immunoglobulin)
- IO is recessive, two IO alleles must be present for the person to have type O blood
- IA and IB are codominant but both are dominant to Io. If a person receives an IA allele and a IB allele, their blood type is type AB, in which characteristics of both A and B antigens are expressed.
Video: Codominance and the inheritance of blood type