Mendel concluded from his experiments that reciprocal crosses yield the same types of progeny in the same proportions. This is in most cases true, but an important exception occurs with the X and Y sex chromosomes. For example, red–green color blindness in humans is due to mutant alleles of a gene in the X chromosome. Reciprocal crosses do not produce the same types and numbers of progeny. In one type of cross, when a color-blind man mates with a woman who is not color blind, all of the sons and daughters have normal color vision. However, in the reciprocal cross, when a color-blind woman mates with a man who is not color blind, all of the daughters have normal color vision but all of the sons are color blind. For the X and Y sex chromosomes, reciprocal crosses are not equivalent.