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.

What are the possible genotype(s) for pure-breeding red petunias? 

Verified step by step guidance

1. Step 1: Understand the biochemical pathways involved in pigment production. Pathway I involves gene A, which converts a colorless pigment (White₁) to blue pigment. Pathway II involves gene B, which converts a colorless pigment (White₂) to red pigment. Both genes assort independently.
2. Step 2: Recognize that pure-breeding red petunias must synthesize red pigment only. This means gene B must be functional (producing the enzyme for red pigment synthesis), while gene A must be non-functional (unable to produce the enzyme for blue pigment synthesis).
3. Step 3: Determine the genotype for gene A. A non-functional gene A can be represented as 'aa', where 'a' is the recessive allele that does not produce the enzyme for blue pigment synthesis.
4. Step 4: Determine the genotype for gene B. A functional gene B can be represented as 'BB' or 'Bb', where 'B' is the dominant allele that produces the enzyme for red pigment synthesis. Since the petunia is pure-breeding, the genotype must be 'BB'.
5. Step 5: Combine the genotypes for both genes. The possible genotype for pure-breeding red petunias is 'aaBB', where 'aa' ensures no blue pigment is produced and 'BB' ensures red pigment is produced.

Key Concepts

Gene Action and Biochemical Pathways

Gene action refers to the process by which genes influence the production of proteins and enzymes that lead to specific traits, such as flower color. In petunias, two distinct biochemical pathways are responsible for synthesizing flower pigments. Pathway I involves gene A, which converts a colorless pigment to blue, while Pathway II involves gene B, which converts a colorless pigment to red. Understanding these pathways is crucial for determining the genotypes associated with different flower colors.

Independent Assortment

Independent assortment is a fundamental principle of genetics stating that alleles for different genes segregate independently of one another during gamete formation. In the context of petunias, the genes controlling blue and red pigment production assort independently, meaning the inheritance of one color does not affect the inheritance of the other. This principle is essential for predicting the possible genotypes of offspring based on parental genotypes.

Pure-Breeding Genotypes

Pure-breeding genotypes refer to homozygous genotypes that consistently produce offspring with the same phenotype when self-fertilized. For red petunias, the pure-breeding genotype must have two copies of the allele for gene B that produces red pigment, while gene A must be non-functional (homozygous recessive). Therefore, the genotype for pure-breeding red petunias would be 'aaBB', where 'a' represents the non-functional allele of gene A and 'B' represents the functional allele of gene B.