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?

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 loci (A or B) results in blue or purple flowers, while the absence of dominant alleles (homozygous recessive) 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 predicting the phenotypes of offspring in genetic crosses, as it influences the overall expression of traits.

Punnett Square and Phenotypic Ratios

A Punnett square is a tool used in genetics to predict the genotypes and phenotypes of offspring from a genetic cross. By filling out a Punnett square for the F₁ plants, one can determine the expected phenotypic ratios in the F₂ generation. This method allows for the visualization of how alleles segregate and combine, leading to specific flower colors based on the dominant and recessive alleles present.