Which pattern of inheritance occurs when both alleles in a heterozygote are expressed in the phenotype?

Which pattern of inheritance occurs when both alleles in a heterozygote are expressed in the phenotype?

• A. Dominant
• B. Particulate inheritance
• C. Co-dominant
• D. Linkage

Answer: (C)

Co-dominance is when two different alleles in a heterozygote are both expressed in the phenotype. In this case, neither allele masks the other, so you see contributions from both. A classic example is the ABO blood group system: IAIB individuals express both A and B antigens on their red blood cells, yielding the AB blood type. This differs from a simple dominant-recessive pattern, where the dominant allele would hide the other in the heterozygote, giving a single phenotype. It’s also different from incomplete dominance, where the heterozygote shows an intermediate trait rather than both traits simultaneously. The other terms refer to different ideas—dominance describes expression in heterozygotes, while linkage concerns genes that tend to be inherited together, and particulate inheritance is about genes existing as discrete units—so they don’t describe the pattern of dual allele expression seen in co-dominance.

Co-dominance is when two different alleles in a heterozygote are both expressed in the phenotype. In this case, neither allele masks the other, so you see contributions from both. A classic example is the ABO blood group system: IAIB individuals express both A and B antigens on their red blood cells, yielding the AB blood type. This differs from a simple dominant-recessive pattern, where the dominant allele would hide the other in the heterozygote, giving a single phenotype. It’s also different from incomplete dominance, where the heterozygote shows an intermediate trait rather than both traits simultaneously. The other terms refer to different ideas—dominance describes expression in heterozygotes, while linkage concerns genes that tend to be inherited together, and particulate inheritance is about genes existing as discrete units—so they don’t describe the pattern of dual allele expression seen in co-dominance.