In a cross between a black and a white guinea pig, all members of the F₁ generation are black. The F₂ generation is made up of approximately 3/4 black and 1/4 white guinea pigs. Two different matings were made between black members of the F₂ generation, with the following results. Diagram each of the crosses.
Cross Offspring
Cross 1 All black
Cross 2 3/4 black, 1/4 white

Verified step by step guidance

1. Determine the inheritance pattern: The problem indicates a 3:1 ratio in the F₂ generation, which suggests that the black coat color is dominant over the white coat color. This is consistent with Mendelian inheritance for a single gene with two alleles.
2. Assign symbols to the alleles: Let 'B' represent the dominant allele for black coat color and 'b' represent the recessive allele for white coat color. Black guinea pigs can have genotypes 'BB' (homozygous dominant) or 'Bb' (heterozygous), while white guinea pigs must have the genotype 'bb' (homozygous recessive).
3. Analyze Cross 1: The offspring of Cross 1 are all black. This suggests that both parents are homozygous dominant (BB) because a cross between two 'BB' individuals can only produce 'BB' offspring. Diagram the Punnett square for this cross to confirm.
4. Analyze Cross 2: The offspring of Cross 2 show a 3:1 ratio of black to white. This suggests that both parents are heterozygous (Bb). A cross between two 'Bb' individuals produces offspring in the genotypic ratio 1:2:1 (1 'BB', 2 'Bb', 1 'bb') and the phenotypic ratio 3:1 (3 black, 1 white). Diagram the Punnett square for this cross to confirm.
5. Summarize the results: Cross 1 involves two homozygous dominant (BB) parents, resulting in all black offspring. Cross 2 involves two heterozygous (Bb) parents, resulting in a 3:1 phenotypic ratio of black to white offspring. Use the Punnett squares to visually represent these crosses and their outcomes.

Key Concepts

Mendelian Inheritance

Mendelian inheritance refers to the principles of heredity established by Gregor Mendel, which include the concepts of dominant and recessive traits. In this case, the black coat color in guinea pigs is dominant over the white coat color. This means that when a dominant allele is present, it will express the dominant phenotype, as seen in the F₁ generation where all offspring are black.

Punnett Square

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. It helps visualize the combinations of alleles from the parents. In this scenario, constructing Punnett squares for the F₂ generation can illustrate how the 3/4 black and 1/4 white ratio arises from the mating of two heterozygous black guinea pigs (Bb x Bb).

Genotype and Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype is the observable physical or biochemical characteristics. In the guinea pig example, the genotype of the black offspring could be either homozygous dominant (BB) or heterozygous (Bb), but the phenotype is black in both cases. Understanding this distinction is crucial for interpreting the results of the crosses.