Fig. 19. The red and green color pigment gene locus. The red and green color pigment genes lie close together on the X chromosome, separated by only approximately 10 kb. The green gene has been duplicated or in some cases triplicated, presumably through unequal crossing over. Some of these unequal cross-over events lead to color blindness through the deletion of the green gene or the creation of a hybrid red-green gene, which is defective.