Tetragametic chimerism

Tetragametic chimerism is a condition which occurs through the fertilization of two ova by two sperm, followed by the fusion of the zygotes and the development of an organism with intermingled cell lines. This phenomenon happens at a very early stage of development, often during the blastocyst phase. A tetragametic chimera is named such because it is formed from four gametes: two eggs and two sperm. For instance, a chimera can be formed from the merger of two nonidentical twins in a very early (zygote or blastocyst) phase. As such, they can be male, female, or hermaphroditic.

As the chimera organism develops, it may come to possess organs that have different sets of chromosomes. For instance, a chimera may have a heart composed of cells with one set of chromosomes and have a brain composed of cells with a second set of chromosomes. This type of chimerism has occurred in humans and, at one time, was thought to be extremely rare. However, newer evidence seems to indicate that it may not be so rare as once considered. Since phenotypic traits of chimerism are often quite subtle, the majority of chimeras may go through life not realizing their condition.

Chimerism can be diagnosed by the finding of two populations of red cells or, if the zygotes are of opposite sex, ambiguous genitalia and hermaphroditism alone or in combination. Sometimes, affected persons may also have patchy skin, hair, or eye pigmentation (heterochromia). If the blastocysts are of the same sex, the condition can only be detected through DNA testing. If the blastocysts are of the opposite sex, genitals of both sexes may be formed, thereby exhibiting hermaphroditism. In 2003, New Scientist reported that there were approximately 30-40 documented human cases in the literature.

Naturally occurring chimeras are rarely identified unless their offspring have abnormalities such as male/female or hermaphrodite characteristics or skin discolouring (visible Blaschko's lines). However, chimerism can be detected in DNA testing. A landmark chimera case, the Lydia Fairchild case of 2002, was brought to court after DNA testing showed that her children could not be hers, since their DNA did not match. She was charged with fraud, but the charges against her were dismissed when it became clear that she was a chimera, with the matching DNA being found in her cervical tissue.