Black and brown guinea pigs are bred together. Assuming their color is determined by one gene, and that there is a dominant and a recessive allele, how would you determine which allele is dominant? How could you determine the genotypes of the parents? Explain.

Question

What will be an ideal response?

Answer 1

Answers may vary.
Situation 1: If both parents are homozygous and the black allele is dominant, all F1 offspring will have a black color. The F2 offspring will have an average phenotypic ratio of 3 black: 1 brown.
Situation 2: If both parents are homozygous and the brown allele is dominant, all F1 offspring will have a brown color. The F2 offspring will have an average phenotypic ratio of 3 brown: 1 black.
Situation 3: If one parent is homozygous recessive and one is heterozygous, then an average of half the F1 offspring will be black and half will be brown. An average of half will have a homozygous recessive genotype and half will have a heterozygous genotype. To determine which allele is dominant, I would breed F1 offspring of the same color (i.e. two browns and two blacks). F1 parents expressing the recessive trait would have a homozygous recessive genotype and produce offspring of all the same phenotype. Two brown-colored guinea pigs would produce all browns or two blacks would produce all blacks. When two heterozygous parents are crossed, the dominant phenotype will be expressed in 3/4 of the offspring and only 1/4 of the offspring will express the recessive trait. Therefore, I would study the F2 phenotypes of both crosses to determine the dominant and recessive traits.
Situation 4: I might also breed black-colored guinea pigs from the parent generation with other black guinea pigs and brown-colored guinea pigs with other brown guinea pigs. If they are homozygous, the color of offspring will remain the same as the parent color through the generations.