Blood group genes are inherited in a straightforward manner and their products are readily detectable. Because of this they are utilized over a wide area of scientific endeavor and provide excellent markers for use in gene mapping of the human chromosomes. Both the Rh and Duffy blood group genes have now been localized to the number 1 chromosome. Studies of a patient whose red cells exhibit mosaicism for the Rh blood group indicate that loss of the end of the short arm of chromosome number 1 is associated with loss of an Rh gene complex. The Rh gene can, therefore, be assigned with some precision to this region of the chromosome.