Final answer:
The rarest blood type combination in the ABO and Rh blood groups is O negative. It lacks A and B antigens and the Rh D antigen, making it the universal donor but also the least common.
Step-by-step explanation:
The rarest blood type combination based on ABO and Rh blood groups is generally O negative (O-). The ABO blood group system determines the presence of antigens A and B on the erythrocytes. A person can have:
- Type A blood, with only the A antigen present.
- Type B blood, with only the B antigen present.
- Type AB blood, with both A and B antigens present.
- Type O blood, which lacks both A and B antigens.
Separately, the Rh blood group system is determined by the presence or absence of the Rh D antigen, with those lacking the antigen being Rh negative, and this is rarer compared to Rh positive. Since O negative blood lacks both A and B antigens as well as the Rh antigen, it is considered the universal donor but is also the rarest.