This condition is not a science fiction. It happens in real life. Please see this Wikipedia article: Lydia Fairchild – Wikipedia. A woman is denied her already established motherhood because of a negative genetic test. The truth is that the woman is a chimera of two fraternal twins. She is two persons in one.
Many may live with this condition, but not identified. No one would suspect any “genetic abnormality” unless there was some striking observation, e.g., having two different-colored eyes or having male and female organs. It is said that the incidence of such a condition should be increasing due to the increased use of fertility drugs and increased twin conception.