Talk about the Rhesus group.

The Rh factor is also inherited from the parents but independently of the ABO blood type alleles. There are two different alleles for the Rh factor known as: Rh+ (or R or Rh or D) dominant and Rh- (or r or rh or d) recessive. Recessive rr individuals produce no Rh antigens and are Rh negative, while RR and Rr individuals have Rh antigens on the surface of their red blood cells and are classified as Rh positive. The function of Rh antigens remains unknown, but we suggest that they may be involved in maintaining the structural and functional properties of the erythrocyte membrane.