Question

When both alleles express their effect on being present together, the phenomenon is called

• A. dominance
• B. codominance
• C. pseudodominance
• D. amphidominance

Answer 1

Answer: (B)

When both alleles express their effect on being present together, the phenomenon is called codominance. As a result the heterozygous condition has a phenotype different from either of homozygous genotypes, e.g. AB blood group in humans. Alleles for blood group $A(I_A)$ and blood group $B(I_B)$ are codominant so that when they come together in an individual, they produce blood group AB. It is characterised by the presence of both antigen A (from $I_A$) and antigen B ($I_B$) over the surface of erythrocytes.