Which inheritance pattern occurs when the heterozygote expresses both parental traits simultaneously?

Which inheritance pattern occurs when the heterozygote expresses both parental traits simultaneously?

• A. Incomplete dominance
• B. Monohybrid
• C. Complete dominance
• D. Co-dominance

Answer: (D)

When a heterozygote expresses both parental traits at the same time, the alleles are co-dominant. In this situation, neither allele masks the other, so both phenotypic effects appear in the organism. A classic example is the AB blood type, where the IA and IB alleles are both expressed, giving A and B antigens on the surface of red blood cells. Another example is a roan coat, where both red and white hairs are present rather than one color blending into the other. This is different from incomplete dominance, where the heterozygous phenotype is a blend (like pink from red and white), and from complete dominance, where one allele completely masks the other in the organism. A monohybrid designation refers to a cross involving one gene, not to how the trait is expressed in the heterozygote.

When a heterozygote expresses both parental traits at the same time, the alleles are co-dominant. In this situation, neither allele masks the other, so both phenotypic effects appear in the organism. A classic example is the AB blood type, where the IA and IB alleles are both expressed, giving A and B antigens on the surface of red blood cells. Another example is a roan coat, where both red and white hairs are present rather than one color blending into the other. This is different from incomplete dominance, where the heterozygous phenotype is a blend (like pink from red and white), and from complete dominance, where one allele completely masks the other in the organism. A monohybrid designation refers to a cross involving one gene, not to how the trait is expressed in the heterozygote.