Codominance means both alleles are fully expressed in the heterozygote. Which human example demonstrates codominance?

• A. AB blood group (IAIB)
• B. Blood type O
• C. Incomplete dominance for pink flowers
• D. XY sex determination

Answer: (A)

Codominance is when the two alleles in a heterozygote are both expressed, giving a phenotype that shows traits from both alleles. The AB blood group is the clear human example: with IAIB, both A and B antigens are produced on the surface of red blood cells, so the individual has AB blood type. This shows both alleles being expressed rather than one dominating or blending.

In contrast, blood type O (genotype ii) lacks A and B antigens because neither allele expresses them, so there’s no codominant expression. Incomplete dominance would give a blended, intermediate trait—like pink flowers from red and white—not the simultaneous display of both traits. XY sex determination isn’t about one gene’s two alleles being expressed together in this way, so it doesn’t illustrate codominance.