Feather color in parakeets is produced by the blending of pigments from two biosynthetic pathways shown below. Four independently assorting genes (A, B, C, and D) produce enzymes that catalyze separate steps of the pathways. For the questions below, use an uppercase letter to indicate a dominant allele producing full enzymatic activity and a lowercase letter to indicate a recessive allele producing no functional enzyme. Feather colors produced by mixing pigments are green (yellow + blue) and purple (red + blue). Red, yellow, and blue feathers result from the production of one colored pigment, and white results from the absence of pigment production.

If a pure-breeding blue strain of parakeet (aa BB CC DD) is crossed to one that is pure-breeding purple, predict the genotype(s) and phenotype(s) of the F₁. Show your work

Verified step by step guidance

1. Identify the genotypes of the parental strains: The pure-breeding blue strain has genotype aa BB CC DD, and the pure-breeding purple strain must have dominant alleles for enzymes producing red and blue pigments. Since purple is a mix of red and blue, the purple strain must have at least one dominant allele for Enzyme A (A) and Enzyme B (B) in Pathway I to produce red, and dominant alleles for Enzyme C (C) and Enzyme B (B) in Pathway II to produce blue. Therefore, the purple strain genotype is AA BB CC DD.
2. Determine the F1 genotype by crossing the two parental genotypes: Cross aa BB CC DD (blue) with AA BB CC DD (purple). Since the genes assort independently, the F1 offspring will be heterozygous for gene A (Aa) and homozygous dominant for B, C, and D (BB CC DD).
3. Analyze enzyme activity in the F1: The heterozygous Aa genotype means Enzyme A is functional (dominant allele present), so Pathway I can produce red pigment. The BB genotype means Enzyme B is functional, allowing conversion to yellow in Pathway I and blue in Pathway II. The CC genotype means Enzyme C is functional, so Pathway II can produce the blue pigment. The DD genotype is also functional but not directly involved in pigment production here.
4. Predict the pigments produced in the F1: Pathway I produces red pigment (due to functional Enzyme A and B), and Pathway II produces blue pigment (due to functional Enzyme C and B). The combination of red and blue pigments results in purple feathers.
5. Summarize the F1 phenotype and genotype: The F1 genotype is Aa BB CC DD, and the phenotype is purple feathers, resulting from the combination of red and blue pigments produced by the two biosynthetic pathways.

Key Concepts

Independent Assortment of Genes

Independent assortment refers to the principle that genes located on different chromosomes are inherited independently of each other. In this question, genes A, B, C, and D assort independently, meaning the alleles for each gene segregate without influencing the others, which affects the combination of enzymes produced and thus pigment synthesis.

Enzyme Function and Allelic Dominance

Each gene codes for an enzyme that catalyzes a step in pigment biosynthesis. Dominant alleles (uppercase) produce functional enzymes, while recessive alleles (lowercase) produce nonfunctional enzymes. The presence or absence of enzyme activity determines which pigments are synthesized, influencing feather color.

Biosynthetic Pathways and Pigment Blending

Two separate biosynthetic pathways produce pigments that combine to form feather colors. Pathway I produces red and yellow pigments, while Pathway II produces blue pigment. The mixing of pigments (yellow + blue = green, red + blue = purple) explains the observed feather colors based on enzyme activity in each pathway.