Question

In a testcross of a tall, yellow, round plant with a homozygous recessive dwarf, green, wrinkled plant, the frequency of tall, yellow, round plants was 1/4. How many traits was the tall, yellow, round plant heterozygous for? (Assume tall, yellow, and round are the dominant traits.)

one trait
two traits
all three traits
none of the traits

Answer 1

The tall, yellow, round pea plant was heterozygous for all three traits as indicated by the 1/4 frequency of tall, yellow, round offspring from a testcross with a homozygous recessive plant.

Step-by-step explanation:

In a testcross involving a tall, yellow, round pea plant, if the frequency of offspring that are also tall, yellow, and round is 1/4, we can determine the genotype of the tall, yellow, round plant. Considering that the recessive phenotypes all appeared together, and given the dominant and recessive inheritance patterns, we can assume independent assortment with a 9:3:3:1 ratio collapsed into two 3:1 monohybrid ratios. However, since we see a 1/4 frequency for the dominant phenotype, the tall, yellow, round plant must be heterozygous for all three traits. This is because the probability of an offspring being tall, yellow, and round from a heterozygote crossed with a homozygous recessive (dwarf, green, wrinkled) is (1/2) × (1/2) × (1/2) = 1/8 for each dominant trait, which when combined, gives 1/4.