In rat, black coat color is dominant over white coat color. if some of the offspring of a cross between ablack rat and a white rat are white , the black must have been

It must've been the recessive gene. A dominant gene is what the offspring gets. And the recessive is too weak so the offspring doesn't. For example, if a mother has blue eyes and a father has green eyes, what will the child have? If the blue was the dominant gene and the green was recessive, the child would have blue eyes.