Question

Which of the following is the example of codominance?

• A. $H{b}^{A}H{b}^A,I^A{I}^B$
• B. $H{b}^{S}H{b}^S,I^A{I}^B$
• C. $H{b}^{A}H{b}^S,I^A{I}^B$
• D. $H{b}^{S}H{b}^S,I^A{I}^A$

Answer 1

The correct option is C $H{b}^{A}H{b}^S,I^A{I}^B$

Generally, the dominant allele is expressed and the effect of the other recessive allele is masked. But when there are two alleles that are dominant and expressed simultaneously, the effect is known as co-dominance. For the $\beta$- globin chain of haemoglobin, there are three molecular phenotypes of Hb${}^A$/Hb${}^A$, Hb${}^A$/Hb${}^S$ and Hb${}^S$/Hb${}^S$. These alleles are present at different loci and are co-expressed. The carrier for the sickle cell trait will express both the alleles Hb${}^A$/Hb${}^S$. Similarly, for the individual with AB blood group, both the alleles I${}^A$ and I${}^B$ will be expressed.

Thus, the correct answer is option C.