In the fantastic Jillalope, the offspring of a true-breeding black parent and a true-breeding white parent all appear gray. When these gray offspring are crossed among themselves, their offspring always appear in a ratio of 1 black : 2 gray : 1 white. Upon close examination of the coats with a magnifier, each hair of a gray animal has closely-spaced alternating black spots and white spots, but no gray spots. What is the mode of inheritance