Advanced Placement (AP) Biology Practice Exam

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What characterizes incomplete dominance in genetics?

One allele completely masks the other
Two alleles produce separate traits
Alleles blend their traits together
Only one allele is present in the organism

The correct answer is: Alleles blend their traits together

Incomplete dominance is characterized by the blending of traits from two different alleles. In this genetic scenario, when an organism inherits two different alleles for a specific trait, neither allele is dominant over the other. Instead of one trait completely masking the other, the heterozygous phenotype displays an intermediate expression of the traits. For example, if one allele codes for red flower color and another for white flower color, the offspring may exhibit pink flowers, which is a blend of the two parental traits. This distinct blending effect is what defines incomplete dominance, distinguishing it from complete dominance or co-dominance, where one allele can either dominate completely over the other or both are expressed simultaneously.