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

검증된 단계별 안내

Step 1: Understand the genetic system described. The C allele controls pigment production: C_ (CC or Cc) produces pigment, while cc results in white fur. Among pigmented mice (C_), the A locus determines color: AA or Aa gives agouti, and aa gives black.

Step 2: Analyze the males' genotype: aacc. Since they are cc, they are white and do not produce pigment. Their genotype is homozygous recessive at both loci.

Step 3: Since the males are aacc, all offspring will inherit a c allele from the father, making their genotype at the C locus at least c_. The mother’s genotype is unknown but must have at least one C allele to produce pigment in offspring.

Step 4: For each cross, use the phenotypic ratios to deduce the mother's genotype. For example, if all offspring are pigmented (agouti), the mother is likely CC or Cc and AA or Aa. If white offspring appear, the mother may be Cc, allowing cc offspring (white) to appear.

Step 5: Use Punnett squares for each locus separately (C and A) to predict offspring genotypes and phenotypes. Combine these to match the observed ratios, which will help infer the mother's genotype in each cross.

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주요 개념

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Epistasis and Gene Interaction

Epistasis occurs when one gene masks or modifies the expression of another gene. In this question, the C allele controls pigment production, and its recessive form (cc) results in white fur regardless of the A locus genotype. Understanding how the C locus epistatically affects the A locus is essential to interpret the phenotypic ratios.

Mendelian Inheritance and Genotypic Ratios

Mendelian inheritance principles explain how alleles segregate and assort independently during gamete formation. Knowing how to predict offspring genotypes and phenotypes from parental crosses, especially with dominant and recessive alleles, helps analyze the given crosses and their resulting phenotypic ratios.

Phenotypic Ratios and Genotype Inference

Phenotypic ratios observed in offspring can be used to infer the unknown parental genotypes. By comparing expected ratios from different genotype combinations with observed data, one can deduce the likely genotypes of the agouti females, considering the interaction between the A and C loci.