Codominance
For some genes, there are more than two possible alleles. Codominance means that 2+ alleles are dominant and
both will be expressed in an organism. When we talk about codominance, we represent the alleles using one
capital letter for the gene, with different superscript letters for each allele:
CWCw or CDCd or CPCp
The best example of codominance is human blood types.
Human blood types are controlled by multiple alleles of the same gene. The gene responsible for blood type is “I”
The three alleles of the I gene are IA IB and i
● i is recessive to I
● IA and IB are codominant
These are the genotypes for blood types
Type A blood = IAIA or IAi
Type AB = IAIB
Type B blood = IB IB or IB i
Type O = ii
Example:
A father with Type A blood and a mother with Type B blood have 4 kids. The children have blood types A, B,
AB, and O respectively. What must the genotype of the children be?