Suppose that tail length in horses is controlled by a number of additive genes, each of which has 2 alleles. When a true-breeding female with a 12-inch tail is mated to a true-breeding male with a 36-inch tail, all F1 offspring have the same tail length. When the F1 offspring are mated to each other, 7 different tail lengths are seen in the F2 offspring. How many genes contribute to tail length?