When Mrs. McK., 25, gave a pint of blood for Britain's National Blood Transfusion Service in Sheffield last March, the doctor and nurses who checked her saw nothing unusual. But when technicians typed the blood, they did a double take, and with good reason: Mrs. McK. was the first human being in medical history with a double set of blood groups. Her red cells were 61% type 0 and 39% type A.
Puzzled, the researchers asked Mrs. McK. whether she was a twin. No less puzzled by their apparent second sight, Mrs. McK. replied that she had had a twin brother, who...
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