Three independently assorting genes (A, B, and C) are known to control the following biochemical pathway that provides the basis for flower color in a hypothetical plant:

Three homozygous recessive mutations are also known, each of which interrupts a different one of these steps. Determine the phenotypic results in the F₁ and F₂ generations resulting from the P₁ crosses of true-breeding plants listed here:
yellow (AAbbCC) × green (AABBcc)

Verified step by step guidance

1. Step 1: Understand the biochemical pathway and gene functions. The pathway is: colorless \( \xrightarrow{A-} \) yellow \( \xrightarrow{B-} \) green \( \xrightarrow{C-} \) speckled. Each gene (A, B, C) controls a step, and the presence of at least one dominant allele (A-, B-, C-) allows progression to the next color. Homozygous recessive mutations (aa, bb, cc) block the pathway at that step, resulting in the color corresponding to the last completed step.
2. Step 2: Identify the genotypes of the parental plants. The yellow parent is \( AAbbCC \), meaning it has dominant A and C alleles but is homozygous recessive for gene B (bb), so the pathway stops at yellow. The green parent is \( AABBcc \), meaning it has dominant A and B alleles but is homozygous recessive for gene C (cc), so the pathway stops at green.
3. Step 3: Determine the F1 genotype by crossing \( AAbbCC \) (yellow) with \( AABBcc \) (green). For each gene, combine alleles: gene A will be \( Aa \), gene B will be \( Bb \), and gene C will be \( Cc \). Since all are heterozygous dominant, the F1 plants will have the genotype \( AaBbCc \).
4. Step 4: Predict the F1 phenotype. Since all genes are heterozygous dominant, the pathway proceeds through all steps: colorless to yellow to green to speckled. Therefore, the F1 phenotype will be speckled.
5. Step 5: Analyze the F2 generation by self-crossing the F1 \( AaBbCc \) plants. Use a Punnett square or the product rule to determine the genotypic and phenotypic ratios. Each gene segregates independently with a 1:2:1 genotype ratio and 3:1 dominant to recessive phenotype ratio. Combine these to find the proportions of plants showing colorless, yellow, green, and speckled phenotypes based on which gene's recessive homozygous genotype blocks the pathway.

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, and C assort independently, meaning the alleles for each gene segregate without influencing the others, which affects the genotypic and phenotypic ratios in offspring.

Biochemical Pathway and Epistasis

The biochemical pathway shows a sequential conversion from colorless to yellow, green, and speckled flowers, controlled by genes A, B, and C respectively. Mutations in any gene can block the pathway, causing epistasis where one gene's effect masks others, influencing the final flower color phenotype.

Genotypic and Phenotypic Ratios in F1 and F2 Generations

Crossing true-breeding parents with different homozygous genotypes produces predictable F1 heterozygotes. The F2 generation, from self-crossing F1, shows segregation and recombination of alleles, resulting in various genotypic and phenotypic ratios based on Mendelian inheritance and the biochemical pathway.