Blue flower color is produced in a species of morning glories when dominant alleles are present at two gene loci, A and B. (Plants with the genotype have blue flowers.) Purple flowers result when a dominant allele is present at only one of the two gene loci, A or B. (Plants with the genotypes and are purple.) Flowers are red when the plant is homozygous recessive for each gene (i.e., aabb).
If two F₁ plants are crossed, what are the expected phenotypes and frequencies in the F₂?

Verified step by step guidance

1. Step 1: Understand the genetic basis of the problem. The flower color is determined by two gene loci, A and B. Blue flowers occur when at least one dominant allele is present at both loci (A_B_). Purple flowers occur when a dominant allele is present at only one locus (A_bb or aaB_). Red flowers occur when the plant is homozygous recessive at both loci (aabb).
2. Step 2: Determine the genotypes of the F₁ plants. Since the F₁ plants are the result of a cross between two true-breeding parents (e.g., AABB x aabb), the F₁ plants will all have the heterozygous genotype AaBb.
3. Step 3: Set up a Punnett square to determine the genotypes of the F₂ generation. Since the F₁ plants are AaBb, you will need to create a 4x4 Punnett square to account for all possible combinations of alleles from the two loci (A and B). The rows and columns of the Punnett square will represent the gametes (AB, Ab, aB, ab) produced by the F₁ plants.
4. Step 4: Identify the phenotypes corresponding to each genotype in the Punnett square. Use the rules provided in the problem: A_B_ = blue, A_bb or aaB_ = purple, and aabb = red. Count the number of occurrences of each phenotype.
5. Step 5: Calculate the expected phenotypic ratios in the F₂ generation. Divide the number of occurrences of each phenotype by the total number of offspring (16, since there are 16 cells in the Punnett square). This will give you the expected frequencies of blue, purple, and red flowers.

Key Concepts

Dominant and Recessive Alleles

In genetics, alleles are different forms of a gene that can exist at a specific locus. Dominant alleles mask the effect of recessive alleles in heterozygous conditions. In the case of the morning glories, the presence of dominant alleles at either or both gene loci (A or B) results in blue or purple flowers, while the homozygous recessive condition (aabb) leads to red flowers.

Gene Loci and Epistasis

Gene loci refer to the specific locations of genes on a chromosome. In this scenario, two loci (A and B) interact to determine flower color, demonstrating a form of epistasis where the expression of one gene affects the expression of another. This interaction is crucial for understanding the phenotypic outcomes when different genotypes are crossed.

Punnett Square and Phenotypic Ratios

A Punnett square is a tool used in genetics to predict the genotypic and phenotypic ratios of offspring from a genetic cross. By crossing the F₁ plants, one can determine the expected frequencies of blue, purple, and red flowers in the F₂ generation. The ratios derived from the Punnett square will reflect the dominance relationships and the combinations of alleles inherited from the parents.