Strains of petunias come in four pure-breeding colors: white, blue, red, and purple. White petunias are produced when plants synthesize no flower pigment. Blue petunias and red petunias are produced when plants synthesize blue or red pigment only. Purple petunias are produced in plants that synthesize both red and blue pigment (the mixture of red and blue makes purple). Flower-color pigments are synthesized by gene action in two separate pigment-producing biochemical pathways. Pathway I contains gene A that produces an enzyme to catalyze conversion of a colorless pigment designated  to blue pigment. In Pathway II, the enzymatic product of gene B converts the colorless pigment designated  to red pigment. The two genes assort independently.

True-breeding red petunias are crossed to pure-breeding blue petunias, and all the F₁ progeny have purple flowers. If the F₁ are allowed to self-fertilize and produce the F₂, what is the expected phenotypic distribution of the F₂ progeny? Show your work.

Key Concepts

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, as well as the segregation and independent assortment of genes during gamete formation. In this context, the inheritance of flower color in petunias follows Mendelian principles, where the alleles for blue and red pigments are dominant over the allele for no pigment.

Phenotypic Ratios

Phenotypic ratios describe the relative frequencies of different observable traits in the offspring of a genetic cross. In the case of the petunias, the F₁ generation produces purple flowers due to the combination of red and blue pigments. When the F₁ plants self-fertilize, the expected phenotypic ratio in the F₂ generation can be predicted using a Punnett square, which will reveal the distribution of purple, red, and blue flowers.

Independent Assortment

Independent assortment is a principle stating that alleles for different genes segregate independently of one another during gamete formation. In this scenario, the genes controlling flower color in petunias (gene A for blue and gene B for red) assort independently, allowing for a variety of combinations in the F₂ generation. This concept is crucial for predicting the expected phenotypic distribution of the offspring when the F₁ generation is self-fertilized.