You are given two true-breeding strains of hamster. One strain has white fur color and the other has a dark brown fur color. When you cross the white-fur strain to the dark-brown-fur strain, you obtain F1 progeny that have a light brown fur color

Question

When you cross the F1 progeny with each other, 25% of the F2 generation have white fur, 25% have dark brown fur, and 50% have light brown fur. How many genes crucial for fur coloration differ between the two starting strains? Explain your answer.

Extra credit: Propose a molecular mechanism for how fur color is determined in this species of hamster.

Answer 1

One gene involved in determining fur color differs between these two strains. The white allele is not fully recessive to the dark brown allele. Therefore, any animal that is heterozygous (and has one white allele and one dark brown allele) will have a light brown fur color. Thus, the white allele and the dark brown allele are co-dominant. When two heterozygous animals are crossed together, 25% of the offspring will be homozygous for one allele, 25% will be homozygous for the other allele, and 50% will be heterozygous, exactly what is observed in the cross you performed.

Extra credit: Any reasonable answer is fine. There are many possible answers, including the following.

1. The dark brown and white alleles are alleles of a gene involved in synthesizing pigment. Hamsters with the white allele do not produce any pigment; hamsters with the dark brown allele produce pigment. When hamsters are heterozygous, they have one chromosome that carries the white allele and does not produce any pigment. Therefore, they produce half the amount of pigment needed normally and thus are light brown instead of dark brown.
2. The dark brown and white alleles are alleles of a gene involved in a microtubule motor that is important for transporting the pigment to the correct place in the cell. Hamsters with the white allele produce a defective version of the microtubule motor and thus no pigment is properly transported, leading to white fur. The hamsters with the dark brown allele produce a properly functioning microtubule motor. When hamsters are heterozygous, they have one chromosome that carries the white allele and one chromosome that carries the dark brown allele; therefore, only half the amount of functioning microtubule motor is being produced in the heterozygote. Because the number of microtubule motors is limiting in the cell, half the number of properly functioning microtubule motors can only transport half the amount of pigment, leading to a light brown fur.