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:
speckled (AABBCC) × yellow (AAbbCC)

Verified step by step guidance

1. Step 1: Identify the genotypes of the parent plants and their phenotypes based on the biochemical pathway. The speckled parent has genotype AABBCC, meaning all dominant alleles are present, so the pathway proceeds fully to speckled. The yellow parent has genotype AAbbCC, meaning it is homozygous recessive for gene B, which interrupts the step from yellow to green, so the phenotype is yellow.
2. Step 2: Determine the genotype of the F1 generation by crossing the two parents. Since the speckled parent is AABBCC and the yellow parent is AAbbCC, the F1 offspring will inherit one allele from each parent for each gene, resulting in genotype AABbCC.
3. Step 3: Predict the phenotype of the F1 generation based on their genotype. Since gene B is heterozygous (Bb) and gene B controls the step from yellow to green, and assuming B is dominant, the pathway proceeds from colorless to yellow to green, but not to speckled because gene C is homozygous dominant (CC). Therefore, the F1 phenotype will be green.
4. Step 4: Determine the possible gametes produced by the F1 individuals for the F2 generation. Since the F1 genotype is AABbCC, the gametes will be AB C and AB c, but since gene A and C are homozygous dominant, only gene B will segregate, producing gametes with B or b alleles.
5. Step 5: Use a Punnett square to predict the genotypic and phenotypic ratios of the F2 generation by crossing F1 individuals (AABbCC × AABbCC). Analyze the combinations of B alleles (BB, Bb, bb) to determine the phenotypes: BB and Bb will allow the pathway to proceed to green or speckled, while bb will interrupt the pathway at yellow. Then, consider gene C to determine if the final color is green or speckled.

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 Pathways and Gene Function

The biochemical pathway shows a sequential conversion of colorless to speckled via yellow and green, controlled by genes A, B, and C respectively. Each gene's functional allele enables a step in the pathway, while recessive mutations block that step, resulting in accumulation of the previous color and affecting the phenotype.

Genotypic and Phenotypic Ratios in Crosses

Understanding how to predict F1 and F2 generation phenotypes requires knowledge of Mendelian genetics, including how homozygous and heterozygous genotypes combine. The cross between speckled (AABBCC) and yellow (AAbbCC) plants will produce specific genotypic combinations that determine the flower color phenotypes in subsequent generations.