What is codominance?

• A. Both alleles in a heterozygote are fully expressed, producing distinct phenotypes
• B. One allele completely masks the other
• C. Heterozygotes show an intermediate phenotype
• D. Expression of alleles depends on environmental conditions

Answer: (A)

Codominance means that both alleles in a heterozygous individual are fully expressed, so you see both traits distinctly rather than a blend. A classic example is the AB blood type, where the IA and IB alleles are both expressed to produce A and B antigens on red blood cells. This differs from complete dominance, where one allele masks the other; and from incomplete dominance, where the heterozygote shows an intermediate phenotype (like pink flowers from red and white). It’s also not about environmental conditions shaping expression.