Rizzoli & Isles Season 2 Episode 2

Goof: Dr. Maura Isles determines that the murder victim could not be baby John Doe's biological mother because of her blood type. She states that the baby's blood type is O and the victim's is A and that it is therefore impossible for her to be his mother. This is a false conclusion.

Blood type is inherited and determined by the presence or absence of A and B antigens on the surface of red blood cells. A and B are dominant traits while O is recessive and represents the absence of either antigen.

If the father is type A, B or O then the child could be hers. The only way to conclude that it would not be possible would be if Dr. Isles knew that the father's blood type was AB. At this point in the episode, Dr. Isles did not know who the baby's father was.