Free Codominance OCR A Level Biology revision notes – covering specification point 6.1.2(bi).
When two alleles at the same gene locus on two homologous chromosomes are different, but neither is dominant over the other, then both will be expressed.
When both alleles in a genotype can express their own phenotype, then it is known as ‘codominance’.
Worked Example
Coat colour in shorthorn cattle is determined by a single gene with two codominant alleles:
- CR produces red hair.
- CW produces white hair.
The three possible genotypes each produce their own distinct phenotypes:
- CRCR cattle have a red coat.
- CWCW cattle have a white coat.
- CRCW cattle have a roan coat, with both red and white hairs present.
It is important to note that the roan coat can be mistaken for an intermediate phenotype produced by incomplete dominance, but is in fact an example of codominance as both alleles are fully expressed, and the visual mixture of red and white hairs gives the illusion of a blended phenotype.
Two roan shorthorn cattle, which are heterozygous, are crossed:
