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 an F₁ plant is backcrossed to one of the pure-breeding parental plants, what is the expected ratio of phenotypes among progeny? Why is the phenotype ratio the same regardless of which parental strain is selected for the backcross?

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 expression of recessive alleles in heterozygous conditions. In the context of the morning glories, the presence of dominant alleles at loci A and B results in blue flowers, while the absence of these dominant alleles leads to purple or red flowers, depending on the genotype.

Gene Interaction

Gene interaction occurs when two or more genes influence a single trait, leading to a variety of phenotypes. In this case, the interaction between the two loci (A and B) determines the flower color in morning glories. The presence of dominant alleles at both loci results in blue flowers, while the presence of a dominant allele at only one locus results in purple flowers, illustrating how multiple genes can affect a single phenotype.

Backcrossing and Phenotypic Ratios

Backcrossing involves crossing an F₁ hybrid with one of its parental strains to analyze the inheritance patterns of traits. The expected phenotypic ratio among progeny from this backcross remains consistent regardless of which parental strain is used because the genetic contributions from the F₁ hybrid and the pure-breeding parent are the same, leading to predictable ratios based on Mendelian inheritance principles.