In corn, three dominant alleles called A, C, and R must be present to produce colored seeds. Genotype A/-; C/-; R / - is colored; all others are colorless. A colored plant is crossed with three tester (homozygous) plants of known genotypes. With tester a/a; c/c; R/R the colored plant produces 50% colored seeds; with the tester a/a C/C r/r it produces 25% colored; and with A/A c/c r/r it produces 50 percent colored. What is the genotype of the colored plant?

The genotype of the colored plant is A/a C/C R/r.

Explanation:

The genotype A/-; C/-; R / - is colored; all others are colorless.

A colored plant (A/?; C/?; R/?) is crossed with three tester.

We know the colored plant can't be homozygous recessive for any gene. When a homozygous dominant genotype is crossed with a homozygous recessive (BB x bb), all the seeds will have one dominant allele (Bb) and therefore color. When a heterozygous genotype for a single gene (Bb) is crossed with a homozygous recessive (bb), 50% of the seeds will be Bb (colored).

1. The cross A / - C / - R / - X a/a c/c R/R produces 50% colored seeds.

All the seeds will have the R allele, so the original plant is heterozygous for either A/a or C/c.

2. The cross A / - C / - R / - X A/A c/c r/r produces 50% colored seeds.

All the seeds will have the A allele, so the original plant is heterozygous for either C/c or R/r.

3. The cross A / - C / - R / - X a/a C/C r/r produces 25% colored seeds.

All the seeds will have the C allele, so the original plant is heterozygous for both A/a and R/r (the offspring would be 1/2 Aa x 1 CC x 1/2 Rr - --> 1/4 colored seeds).

The genotype of the colored plant is therefore A/a C/C R/r.