Rh antigen follows normal Mendelian inheritance. If 'R' is the allele that produces Rhesus antigen and 'r' is the allele which doesn't, then a person with Rh+ve blood group can be RR or Rr while a person with Rh-ve blood group will always be rr (always homozygous). A person with A blood group may have genotype IaIa or IaIo. A person with AB blood group has genotype IaIb. A person with blood group O has genotype IoIo (always homozygous).