Red-flowering snapdragons are homozygous for allele R1. White-flowering snapdragons are homozygous for allele R2. Heterozygous plants (R1R2) bear pink flowers. What phenotypes should appear among F1 offspring of the crosses listed below? What are the expected probabilities for each phenotype?

a) 1/2 red, 1/2 pink; b) all pink; c) 1/4 red, 1/2 pink, 1/4 white; d) 1/2 white, 1/2 pink

Explanation:

The flower color trait in snapdragons shows incomplete dominance: the heterozygous genotype produces an intermediate phenotype between the two different homozygous genoytpes.

The possible genotypes and phenotypes are:

R1R1 : red R1R2: pink R2R2: white

a. R1R1 X R1R2

The R1R1 individual only produces R1 gametes. The R1R2 parent produces 1/2 R1 gametes and 1/2 R2 gametes.

For that reason, the F1 will be:

1/2 R1R1 (red) 1/2 R1R2 (pink)

b. R1R1 X R2R2

The R1R1 individual only produces R1 gametes. The R2R2 parent only produces R2 gametes.

For that reason, the F1 will be:

100% R1R2 (pink)

c. R1R2 X R1R2

Both parents are heterozygous. This is a monohybrid cross, and from Mendel's Laws we expect the following offspring:

1/4 R1R1 (red) 2/4 R1R2 (pink) 1/4 R2R2 (white) d. R1R2 X R2R2

The R1R2 parent produces 1/2 R1 gametes and 1/2 R2 gametes. The R2R2 individual only produces R2 gametes.

For that reason, the F1 will be:

1/2 R2R2 (white) 1/2 R1R2 (pink)