Advanced Placement (AP) Biology Practice Exam

Which of the following traits is typically an example of incomplete dominance?

1. Straight hair versus curly hair

2. Red flowers crossed with white flowers producing pink flowers

3. Brown eyes versus blue eyes

4. Type A and B blood types

The correct answer is: Red flowers crossed with white flowers producing pink flowers

The trait that best exemplifies incomplete dominance is when red flowers crossed with white flowers produce pink flowers. Incomplete dominance occurs when the offspring's phenotype is a blend of the parental traits, rather than one trait being fully expressed over the other. In this case, neither the red nor the white flower allele is completely dominant; instead, the resulting phenotype (pink flowers) represents a mixture of both traits. This stands in contrast to the other choices. For example, straight hair versus curly hair follows a pattern of complete dominance where one allele (curly hair) fully expresses itself over the other. Brown and blue eyes are a result of complete dominance as well, based on the presence of melanin, where brown is dominant. Similarly, blood types A and B represent co-dominance, where both A and B alleles can be expressed simultaneously, resulting in type AB blood rather than a blend like pink flowers.