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. Identify the genotypes and phenotypes involved: The first two genes (A and B) determine coat color with the following phenotypes: A-B- (gray), A-bb (yellow), aaB- (black), and aabb (cream). The third gene (C) controls whether color is expressed: CC or Cc allows color, while cc results in albino regardless of A and B.
2. Determine the possible gametes from each parent: For AaBbCc, the gametes are ABC, ABc, AbC, Abc, aBC, aBc, abC, and abc. For AaBbcc, since cc is homozygous recessive, the gametes are limited to Abc, aBc, abC, and abc (but since cc is fixed, only c alleles are passed).
3. Set up a Punnett square for the C gene first: Cross Cc (from AaBbCc) with cc (from AaBbcc). The offspring genotypes for C will be 50% Cc (color expressed) and 50% cc (albino).
4. For the offspring with Cc genotype, determine the phenotypic ratios based on A and B gene combinations from AaBb × AaBb cross. Use the dihybrid cross ratios for A and B genes to find proportions of gray, yellow, black, and cream.
5. Combine the results: Multiply the proportion of colored offspring (Cc) by the phenotypic ratios from the A and B genes, and add the proportion of albino offspring (cc), which will be 50%. This will give the overall F₁ phenotypic ratio.

Key Concepts

Independent Assortment of Genes

Independent assortment refers to the principle that genes located on different chromosomes segregate independently during gamete formation. This means the alleles of one gene pair segregate without affecting the segregation of another gene pair, allowing for the prediction of genotype and phenotype ratios using Punnett squares.

Epistasis and Gene Interaction

Epistasis occurs when one gene masks or modifies the expression of another gene. In this case, the third gene (C locus) controls whether color is produced at all, overriding the effects of the A and B loci when homozygous recessive (cc), resulting in albino rats regardless of other alleles.

Phenotypic Ratios in Dihybrid and Trihybrid Crosses

Phenotypic ratios describe the proportion of offspring showing particular traits from genetic crosses. When multiple genes interact, such as A, B, and C loci here, calculating phenotypic ratios requires considering both independent assortment and epistatic effects to predict the distribution of coat colors.