Fraternal twins arise from two fertilized eggs (zygotes) that develop into nonidentical siblings. These two zygotes can, on rare occasion, overlap and fuse and develop into one body with two distinct sets of DNA. What is this condition called?

"Karen," as she was presented in the New England Journal of Medicine (Yu et al. 2002) was the mother of three grown children and needed a kidney transplant. During the tissue-match testing, Karen’s doctors insisted that only one of her sons was related to her; the other two were not. Her case caught the attention of the National Institutes for Health (NIH) and she was asked to participate in a study to determine if there was a scientific explanation. After extensive genetic testing from several body tissues, researchers at the NIH determined that Karen was a tetragametic chimera. Instead of the standard twenty-three pairs of chromosomes, she had forty-six. As a result, some of her tissues and organs were genetically mapped to a first set of chromosomes and others to a second set. In essence, she absorbed a fraternal twin in utero and has/had retained both set of chromosomes.