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.


3 brown, 3 yellow, 1 black, 1 cream

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) produces black pigment, while the recessive homozygous genotype (bb) does not. Gene D determines whether yellow pigment is produced. A dominant allele (D) produces yellow pigment, while the recessive homozygous genotype (dd) does not. The combination of both pigments (B- and D-) results in brown coat color, while the absence of both pigments (bbdd) results in cream coat color.
2. Step 2: Analyze the phenotype distribution in the offspring. The phenotypes are distributed as follows: 3 brown, 3 yellow, 1 black, and 1 cream. This suggests a 9:3:3:1 ratio, which is characteristic of a dihybrid cross where both parents are heterozygous for both genes (BbDd).
3. Step 3: Write the possible parental genotypes. To produce a 9:3:3:1 ratio, both parents must have the genotype BbDd. This allows for all possible combinations of alleles (BD, Bd, bD, bd) to be passed on to the offspring.
4. Step 4: Confirm the phenotypic outcomes based on the parental genotypes. Cross BbDd x BbDd and use a Punnett square to determine the offspring genotypes. The resulting genotypes will produce the following phenotypes: brown (B-D-), yellow (bbD-), black (B-dd), and cream (bbdd).
5. Step 5: Match the phenotypic ratios to the observed data. The 9:3:3:1 ratio from the Punnett square matches the observed distribution of 3 brown, 3 yellow, 1 black, and 1 cream. This confirms that the parental genotypes are BbDd.

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. Understanding the relationship between genotype and phenotype is crucial for predicting the traits of offspring based on parental genotypes.

Independent Assortment

Independent assortment is a principle of genetics stating that alleles for different traits segregate independently of one another during gamete formation. This means that the inheritance of one trait (e.g., coat color from gene B) does not affect the inheritance of another trait (e.g., pigment from gene D). This concept is essential for analyzing the phenotypic ratios observed in the offspring.

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 a visual representation of how alleles combine. This tool is particularly useful in determining the expected ratios of different phenotypes, such as brown, yellow, black, and cream coat colors in the given problem.