In shorthorn cattle, the coat colours red or white are controlled by a single pair of alleles. A calf which receives the allele for red coat from its mother and the allele for white coat from its father is called a 'roan'. It has an equal number of red and white hairs in its coat.

(a) Is this an example of codominance or of incomplete dominance?

(b) Give a reason for your answer. 

(c) Give one example in each case of (i) codominance, (ii) incomplete dominance, in humans.

(a) The roan calf exhibits codominance of the two alleles. 

(b) Every allele is fully expressed, i.e. neither allele is dominant.

 (c) (i) the ABO blood groups are instance of the codominance of the IA and the IB alleles.

(ii) Sickle-cell anaemia .is an instance of incomplete dominance. The recessive allele is partially expressed in the heterozygote.