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?

Verified step by step guidance

1. Step 1: Understand the genetic basis of flower color. Blue flowers require dominant alleles at both loci (A and B), purple flowers require a dominant allele at only one locus (A or B), and red flowers occur when the plant is homozygous recessive at both loci (aabb).
2. Step 2: Identify the genotypes of the F₁ plant and the pure-breeding parental plants. The F₁ plant is heterozygous at both loci (AaBb), while the pure-breeding parental plants are either AABB (blue) or aabb (red).
3. Step 3: Determine the possible gametes produced by the F₁ plant (AaBb). The F₁ plant can produce four types of gametes: AB, Ab, aB, and ab, each with equal probability (1/4).
4. Step 4: Analyze the backcross scenarios. If the F₁ plant is backcrossed to the AABB parent, the offspring genotypes will be determined by combining the gametes of the F₁ plant (AB, Ab, aB, ab) with the AABB gametes (AB). Similarly, if the F₁ plant is backcrossed to the aabb parent, the offspring genotypes will be determined by combining the F₁ gametes with the aabb gametes (ab).
5. Step 5: Calculate the expected phenotypic ratios for both backcrosses. In both cases, the phenotypic ratio will be the same because the F₁ gametes (AB, Ab, aB, ab) determine the phenotypes, and the parental gametes (AB or ab) do not alter the phenotypic outcomes. The expected phenotypic ratio among progeny will be 1 blue: 2 purple: 1 red.

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 a hybrid organism (F₁) with one of its parental strains to analyze inheritance patterns. The expected phenotypic ratio among progeny can be predicted using a Punnett square, which shows that the ratio remains consistent regardless of which parental strain is used, due to the fixed genetic contributions from the homozygous parent and the heterozygous F₁ plant.