In rats, gene B produces black coat color if the genotype is , but black pigment is not produced if the genotype is bb. At an independent locus, gene D produces yellow pigment if the genotype is D-, but no pigment is produced when the genotype is dd. Production of both pigments results in brown coat color. If neither pigment is produced, coat color is cream. Determine the genotypes of parents of litters with the following phenotype distributions.


9 black, 7 brown

Verified step by step guidance

1. Step 1: Understand the genetic basis of the problem. Gene B determines whether black pigment is produced. A dominant allele (B) results in black pigment, while the recessive homozygous genotype (bb) does not produce black pigment. Gene D determines whether yellow pigment is produced. A dominant allele (D) results in yellow pigment, while the recessive homozygous genotype (dd) does not produce yellow pigment. The combination of both pigments (B- and D-) results in brown coat color, while the absence of both pigments (bb and dd) results in cream coat color.
2. Step 2: Analyze the phenotype distribution provided in the problem. The phenotypic ratio is 9 black: 7 brown. This suggests a modified Mendelian ratio, which often indicates epistasis (interaction between two genes). Specifically, this ratio suggests complementary gene interaction, where the presence of at least one dominant allele from each gene is required for a specific phenotype.
3. Step 3: Determine the genotypes responsible for each phenotype. Black coat color occurs when the genotype is B- and dd (black pigment is produced, but yellow pigment is not). Brown coat color occurs when the genotype is B- and D- (both black and yellow pigments are produced).
4. Step 4: Infer the parental genotypes based on the phenotypic ratio. To produce a 9:7 ratio, both parents must be heterozygous for both genes (BbDd). This allows for the segregation of alleles in a dihybrid cross, resulting in the observed phenotypic ratio.
5. Step 5: Verify the phenotypic ratio using a Punnett square. Set up a dihybrid cross (BbDd × BbDd) and calculate the genotypic combinations. Identify the phenotypes corresponding to each genotype and confirm that the ratio of 9 black: 7 brown is consistent with the observed data.

Key Concepts

Genotype and Phenotype

Genotype refers to the genetic makeup of an organism, specifically the alleles present at a given locus. Phenotype, on the other hand, is the observable physical or biochemical characteristics of an organism, which result from the interaction of its genotype with the environment. In this question, the phenotypes of coat colors in rats are determined by the genotypes of genes B and D.

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 (like coat color from gene B) does not affect the inheritance of another trait (like pigment from gene D). Understanding this concept is crucial for predicting the phenotypic ratios in offspring based on parental genotypes.

Punnett Square

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. By organizing the possible gametes from each parent, it allows for the visualization of how alleles combine during fertilization. In this scenario, constructing a Punnett square for the genotypes of the parents will help determine the expected ratios of black and brown coat colors in the offspring.