the entire rh blood group system involves multiple antigens and genes. for rh factor testing, however, only the rhesus factor which refers to the rhd antigen specifically is assayed. the rhd gene that codes for the rhd antigen is located on chromosome 1. rhd is a dominant gene, meaning that as long as at least one rhd gene is inherited from either parent, the rhd antigen is expressed. vice versa, if no rhd gene is inherited, no rhd antigen is produced.