In rats, the following genotypes of two independently assorting autosomal genes determine coat color:

A third gene pair on a separate autosome determines whether or not any color will be produced. The CC and Cc genotypes allow color according to the expression of the A and B alleles. However, the cc genotype results in albino rats regardless of the A and B alleles present. Determine the F₁ phenotypic ratio of the following crosses: AaBBCc×AaBBCc

Verified step by step guidance

1. Step 1: Identify the genes involved and their effects. There are three genes: (1) Gene A with alleles A and a, (2) Gene B with alleles B and b, and (3) Gene C with alleles C and c. Gene A and Gene B determine coat color, while Gene C determines whether color is expressed (C_ or cc).
2. Step 2: Determine the phenotypic effects of each genotype. For Gene A and Gene B: A—B— results in gray, A—bb results in yellow, aaB— results in black, and aabb results in cream. For Gene C: CC and Cc allow color expression, while cc results in albino regardless of the A and B alleles.
3. Step 3: Set up the cross AaBBCc × AaBBCc. This involves three loci: (1) A × A, (2) B × B, and (3) C × C. Use a Punnett square for each locus to determine the genotypic ratios for the offspring.
4. Step 4: Combine the results of the Punnett squares for all three loci. Since the genes assort independently, multiply the probabilities of each genotype combination. For example, the probability of Aa (from Gene A), BB (from Gene B), and Cc (from Gene C) is the product of the probabilities of each individual genotype.
5. Step 5: Determine the phenotypic ratio. For each genotype combination, determine the phenotype based on the rules provided (e.g., cc results in albino, A—B— results in gray, etc.). Count the number of offspring for each phenotype and express the ratio.

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, this principle is crucial for predicting the phenotypic ratios resulting from the crosses of the given genotypes.

Genotype and Phenotype

Genotype refers to the genetic constitution of an organism, specifically the alleles present for a particular gene, while phenotype is the observable physical or biochemical characteristics resulting from the genotype. In this question, understanding how the combinations of alleles (genotypes) for the A, B, and C genes influence the coat color (phenotype) of the rats is essential for determining the F₁ phenotypic ratio.

Epistasis

Epistasis is a genetic interaction where the expression of one gene is affected by one or more other genes. In this scenario, the C gene is epistatic to the A and B genes, meaning that the presence of the cc genotype will mask the expression of the coat color determined by the A and B alleles, resulting in albino rats. This concept is vital for accurately calculating the phenotypic ratios in the given genetic crosses.