Two alleles expressed in a heterozygous individual are described by which term?

• A. Dominant
• B. Co-dominant
• C. Blending inheritance
• D. Sickle cell disease

Answer: (B)

Co-dominance is the pattern where both alleles in a heterozygote are fully expressed in the phenotype. This means you can see traits from both alleles at the same time instead of one masking the other. A classic example is the AB blood type, where red blood cells display both A and B antigens because the IA and IB alleles are expressed together. This differs from a dominant-recessive relationship, where one allele hides the other’s effect, and from blending inheritance, where the traits form a single intermediate phenotype rather than showing both directly. Sickle cell disease represents a specific disease caused by a mutation, not the way two alleles are expressed in a heterozygote.