Question

Suppose in a species of petunia, both locus A and locus B can independently determine petal color. At locus A, pink (A) is dominant over white (a). At locus B, pink (B) is also dominant over white (b). If there are dominant alleles at both loci, magenta petals are produced. If an AA BB plant is crossed to an aa bb plant, what is the ratio of magenta‑ to pink‑ to white‑flowered petunias expected in the F2 progeny?

Answer 1

To determine the F2 progeny phenotypic ratio of petunias with magenta, pink, or white flowers, we perform a dihybrid cross using a Punnett square. The F2 phenotypic ratio of magenta, pink, and white flowers is 9:6:1, involving a cross between an AA BB petunia and an aa bb petunia.

Step-by-step explanation:

The question involves a dihybrid cross in which two loci control the flower phenotype of petunias. The cross is between a petunia plant with genotype AA BB, which produces magenta flowers, and a petunia plant with genotype aa bb, resulting in white flowers. To find the ratio of magenta to pink to white flowered petunias in the F2 progeny, we need to perform a Punnett square analysis for a dihybrid cross.

In the F1 generation, all offspring will be Aa Bb, since they receive one dominant and one recessive allele for both loci from their parents. When these F1 plants are self-crossed, the F2 generation will have various combinations of these alleles.

For the F2 generation, the Punnett square yields the following genotypic ratio: 9:3:3:1 for A-B-, A-bb, aaB-, and aabb genotypes, respectively. Since magenta color is produced only in the presence of at least one dominant allele at both loci (A-B-), pink flowers occur when there is a dominant allele at one locus and homozygous recessive alleles at the other locus (A-bb or aaB-), and white flowers occur when both loci are homozygous recessive (aabb). As a result, the phenotypic ratio of magenta to pink to white flowers in the F2 progeny will be 9:3+3:1 or simply 9:6:1.