# 130 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 C^w produces white flowers, while the allele C^R produces red ones. If these alleles show codominance, then the genotypes and phenotypes are:

genotype       phenotype

C^w C^w               white flowers

C^w C^R               pink flowers

C^R C^R               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: I^A, I^B, I^O (the base letter = I stands for immunoglobulin)
• I^O is recessive, two I^O alleles must be present for the person to have type O blood
• I^A and I^B are codominant but both are dominant to I^o. If a person receives an I^A allele and a I^B allele, their blood type is type AB, in which characteristics of both A and B antigens are expressed. 

Because I^O is dominated by both I^A and I^B alleles, a person with blood group A could have the genotype I^A I^O or I^A I^A. This has implication when having children because, if both parents carry the I^O allele, a child could be born with the genotype I^OI^O (blood group O), even though neither of the parents have this phonotype.  

Video:  Codominance and the inheritance of blood type