SAT Subject Biology Practice Question 417: Answer and Explanation
7. 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.
Suppose the original parent plant is cross- pollinated with a plant that has red flowers and non-alternating leaves. All of the resulting offspring have non-alternating leaves. Which of the following is/are true?
I. None of the offspring have orange flowers.
II. The allele for alternating leaves is recessive.
III. Approximately half of the offspring have red flowers.
A. I only
B. II only
C. III only
D. I and III only
E. II and III only
Correct Answer: E
E This question really needs to be broken down a step at a time, and the I, II, III technique applied. First, let's consider the genotype of the parent plant. If it has red flowers, its genotype for flower color must be homozygous (RR). Furthermore, a cross between a plant homozygous for alternating leaves (the original plant) and this plant (with nonalternating leaves) produces plants that all have non-alternating leaves. This tells us two things: First, the allele for non-alternating leaves must be dominant (otherwise all progeny plants would have alternating leaves), and second, the plant with non-alternating leaves must be homozygous (otherwise some progeny plants would have non-alternating leaves, and some would have alternating leaves). Let's look at the Punnett squares. You can draw two separate Punnett squares, one for flower color and one for leaf arrangement:
This is the Punnett square for flower color.
This is the Punnett square for leaf arrangement. N represents non-alternating leaves, and n represents alternating leaves.
Now let's look at option I. Looking at the Punnett square for flower color, we see that some of the offspring will have orange flowers, so option I is false. You can eliminate answer choices A and D. Option II is true (we discussed that above); you can eliminate choice C. Option III is also true. Looking again at the Punnett square for flower color, about half the offspring should have red flowers. Choice B is eliminated.