Pigment in mouse fur is only produced when the C allele is present. Individuals of the cc genotype are white. If color is present, it may be determined by the A, a alleles. AA or Aa results in agouti color, while aa results in black coats. In three crosses between agouti females whose genotypes were unknown and males of the aacc genotype, the following phenotypic ratios were obtained: (1) 8 agouti (2) 9 agouti (3) 4 agouti 8 white 10 black 5 black 10 white

Key Concepts

Alleles and Genotypes

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. In this context, the C allele is responsible for pigment production in mouse fur, while the c allele leads to a lack of pigment, resulting in white fur. The genotype refers to the genetic makeup of an individual, such as CC, Cc, or cc, which determines the phenotype, or observable traits, like fur color.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In the given crosses, the ratios of agouti, white, and black mice indicate how the alleles interact. Understanding these ratios helps in predicting the outcomes of genetic crosses and in determining the genotypes of the parents based on the observed traits.

Epistasis

Epistasis occurs when the expression of one gene is affected by one or more other genes. In this scenario, the presence of the C allele is necessary for any pigment to be produced, while the A and a alleles determine the specific color if C is present. This interaction illustrates how multiple genes can influence a single trait, complicating the inheritance patterns observed in the offspring.