Question

Consider three yellow, round peas, labeled A, B, and C. Each was grown into a plant and crossed to a plant grown from a green, wrinkled pea. Exactly 100 peas issuing from each cross were sorted into phenotypic classes as follows:
A: 51 yellow round, 49 green roundB: 100 yellow, roundC: 24 yellow round; 26 yellow wrinkled; 25 green round; 25 green wrinkledWhat were the genotypes of A, B and C?Use gene symbols of your choosing; be sure to define each one.

Answer 1

Answer:

Let yellow=G and green=g; round=R and wrinkled=r

A: GgRR

B: GGRR

C: GgRr

Explanation:

The question tells us that plants A,B and C are all crossed with the same green, wrinkled pea plant, thus we know that this is a test cross and the green, wrinkled plant is completely recessive (ggrr). Therefore, we know that yellow is dominant over green and round is dominant over wrinkled.

When plant A was put through the test cross all the peas were round, but half the peas were yellow and half were green. From this we can deduce that plant A must have been heterozygous for the yellow trait and homozygous for the round trait.

When plant B was put through the test cross all the peas were yellow and round. From this we can deduce that plant B must have been homozygous for the yellow trait and homozygous for the round trait.

When plant C was put through the test cross every trait was shown in the peas. We can then deduce that plant C must have been heterozygous for the yellow trait and heterozygous for the round trait.