Corrected entry: When Derek is shot and needs a transfusion, they first offer up Sarah as being O negative, the universal donor, but it is turned down as he needs his own blood type, AB positive. John then states, "Test mine," and his is magically AB even though his type could only be A, B or O with his mother as type O. To be AB, his mother would have had to have been A, B or AB. This was required to have John donate to Derek and to allow Sarah to offer as O-, but is factually impossible.
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Correction: There is a genetic mutation called the Bombay mutation, where a person may have type A or B blood in their genotype, but their blood cells don't carry A or B markers. As such, if Sarah had this mutation, and a doctor took her blood and tested it, it would show up as a type O phenotype, but she would still be able to pass on the gene for type A or B blood to her children.