You have four colonies of mice that are independent of each other. Each has a tr

Question

You have four colonies of mice that are independent of each other. Each has a true-breeding mutation resulting in a short tail. For each colony, the short tail phenotype is recessive to the wild-type long-tail phenotype. You do not know whether these four short tail phenotypes are due to mutations in a single gene, or due to mutations in several different genes, and you decide to find out. After performing all possible pairwise crosses of short-tailed mice among the colonies (colony 1 mouse X colony 2 mouse, etc.), you find that all crosses give you long-tailed mice with the wild-type phenotype. How many genes resulting in short tails are represented among these four colonies?

Explanation / Answer

It is given that all the four colonies of mice have true-breeding mutation that resulted in short tail. Therefore, at least four genes must be involved in short tails. The possibility of 1, 2, or 3 genes is ruled out since all these possibilities give rise to mutant short tail in the offspring. Therefore, at least four genes must control the tail length.

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