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 true-breeding blue petunias? 

Key Concepts

Gene Action and Biochemical Pathways

Gene action refers to the process by which genes influence the traits of an organism through the production of proteins, including enzymes. In the context of petunia flower color, two distinct biochemical pathways are involved, each regulated by different genes (gene A and gene B) that convert a colorless precursor into blue and red pigments, respectively.

True-Breeding Genotypes

True-breeding organisms are homozygous for a particular trait, meaning they carry two identical alleles for that trait. For blue petunias, which only produce blue pigment, the genotype must be homozygous for the allele associated with the blue pigment synthesis, specifically the dominant allele of gene A, resulting in the genotype AA.

Independent Assortment

Independent assortment is a principle of genetics stating that alleles for different traits segregate independently of one another during gamete formation. In this case, since the genes for blue and red pigment synthesis assort independently, the genotype for blue petunias does not affect the inheritance of the red pigment gene, allowing for clear determination of the blue petunia genotype.