A black-furred mouse and a white-furred mouse have 78 offspring. The F₁'s are crossed. The F₂ generation is comprised of 1,098 mice with black fur and 735 mice with white fur. Can you determine which phenotype is dominant?

Question

Quizplus · Accepted Answer

A monohybrid cross is used to determine which allele of a gene is dominant. The cross begins with the parental generation.
For this scenario, let's say that the alleles for the different colored fur are X and x.
*Note: Since we do not know which allele is dominant, we will just use these until we can figure it out and assign it to the correct color.
Each parent must be homozygous (having the same) for one of the two alleles. This means that the black furred mouse has both alleles for black fur which could be xx or XX. Similarly, the white furred mouse has both alleles for white fur, which could be xx or XX.
The offspring make up the first filial (F1) generation and all individuals are heterozygous, which means they have one of each allele (Xx).
Then, they carry out a cross between two F1 individuals. F1 crosses will always mean a cross between two heterozygous individuals, so you will be crossing Xx and Xx.
The offspring of an F1 cross will result in 75% of the offspring expressing the dominant trait (either XX or Xx) and 25% of the offspring expressing the recessive trait (xx). Since most of the generation in our example are with black fur, this means that this is the dominant trait. (Which then means that black fur is X and white fur is x.)

A Black-Furred Mouse and a White-Furred Mouse Have 78 Offspring