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: Understand the genetic basis of the problem. The three genes (A, B, and C) control a biochemical pathway for flower color. Each gene has a dominant allele (A, B, C) and a recessive allele (a, b, c). The dominant alleles are required for the pathway to proceed, while the recessive alleles result in a block at their respective steps.
2. Step 2: Analyze the parental genotypes. The first parent (AABBCC) is homozygous dominant for all three genes and produces a speckled phenotype. The second parent (AAbbCC) is homozygous dominant for genes A and C but homozygous recessive for gene B, resulting in a yellow phenotype.
3. Step 3: Determine the F₁ generation genotypes. Perform a cross between the two parents. Each parent contributes one allele for each gene. The F₁ offspring will inherit one dominant allele (A, B, C) from the first parent and one recessive allele (a, b, c) from the second parent, resulting in the genotype AaBbCC for all F₁ individuals.
4. Step 4: Predict the F₁ phenotype. Since all F₁ individuals have at least one dominant allele for each gene (A, B, and C), the biochemical pathway will proceed without interruption, resulting in the speckled phenotype for all F₁ individuals.
5. Step 5: Determine the F₂ generation genotypes and phenotypes. Perform a dihybrid cross for each gene independently (Aa × Aa, Bb × Bb, CC × CC). Use a Punnett square to calculate the genotypic ratios for each gene. Combine the results to determine the phenotypic ratios in the F₂ generation, considering that any homozygous recessive combination (aa, bb, or cc) will block the pathway at its respective step and produce a different phenotype.

Key Concepts

Independent Assortment

Independent assortment is a fundamental principle of genetics stating that alleles for different genes segregate independently of one another during gamete formation. This means that the inheritance of one trait will not affect the inheritance of another trait, allowing for a variety of combinations in offspring. In the context of the question, it implies that the genes A, B, and C will assort independently when determining the phenotypes of the F₁ and F₂ generations.

Homozygous and Heterozygous

Homozygous refers to an organism having two identical alleles for a particular gene, while heterozygous means having two different alleles. In the given cross, the parental plants are homozygous for their respective traits (AABBCC and AAbbCC), which will influence the genotypes of the F₁ generation. Understanding these terms is crucial for predicting the genetic makeup and resulting phenotypes in subsequent generations.

Phenotypic Ratios

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this scenario, analyzing the F₁ and F₂ generations will involve calculating the expected ratios of flower colors based on the combinations of alleles inherited from the parents. This concept is essential for interpreting the results of the genetic cross and understanding how the mutations affect the biochemical pathway for flower color.