Imagine that you caught a female albino mouse in your kitchen and decided to keep it for a pet. A few months later, while vacationing in Guam, you caught a male albino mouse and decided to take it home for some interesting genetic experiments. You wonder whether the two mice are both albino due to mutations in the same gene. What could you do to find out the answer to this question? Assume that both mutations are recessive.

You can cross them. If all offspring are albinos, then both mutations are in the same gene, if not, then mutations are in different genes.

Explanation:

Traits are controlled by genes, which are composed by at least two alleles. Alleles, or mutations, can be dominant, or recessive. To observe a dominant trait, you need at least one dominant allele in the gene, while to observe the recessive trait you need both alleles in the gene to be recessive.

If mutations are in the same gene:

When you cross homozygous dominant individuals, all the offspring will be wild type (non-mutated). When you cross heterozygous individuals, 3/4 of the offspring will be wild type (non-mutated), and 1/4 will express the recessive mutation. Finally, when you cross homozygous recessive individuals, all the offspring will express the recessive mutation.

If mutations are in different genes, then these proportions of the offspring do not apply because alleles do not interact.