SAT Subject Biology Practice Question 414: Answer and Explanation
4. A self-pollinating plant with orange flowers and alternating leaf arrangement produces 47 plants with red flowers and alternating leaves, 103 plants with orange flowers and alternating leaves, and 51 plants with yellow flowers and alternating leaves.
The allele for yellow flower color is
C. incompletely dominant
D. incompletely recessive
Correct Answer: C
C Upon reading this experiment, you should note that the plant with orange flowers produced plants with red flowers, plants with orange flowers, and plants with yellow flowers. The blended phenotype (orange flowers) is a tip-off for incomplete dominance in flower color, and the parent plant, with orange flowers, must have both the red allele and the yellow allele (is heterozygous). The plant produced 25% red flowers, 50% orange flowers, and 25% yellow flowers. If the allele for yellow flower color were recessive, the heterozygous parent plant would have produced 75% orange flowers and 25% yellow flowers (choice A is eliminated). If it were dominant, the resulting offspring would be 75% yellow flowers and 25% orange flowers (choice B is eliminated). There is no such thing as "incompletely recessive" (choice D is eliminated), and codominance between red and yellow would not produce a blended phenotype, it would produce plants that had both red and yellow flowers, but no orange.