In humans, the genes for red-green color blindness (R = normal, r = color-blind) and hemophilia A (H = normal, h = hemophilia) are both X-linked and only 3 map units apart. Suppose a woman has four sons, and two are colorblind and have hemophilia and two have normal color vision and normal blood clotting. What is the probable genotype of the woman? The answer is HR/hr, but I need an explanation.