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?