In addition to designating a mythological monster, a composite organism, a classic novel, and a web browser, chimera also refers to an individual carrying two entirely different DNA profiles. This (as detailed by m_turner) can occur as a result of transplant, transfusion, or mutation, but in these cases the secondary DNA profile tends to be isolated in one organ or system. A more fascinating type, the result of a condition called tetragametic chimerism, exists when an individual develops with two entirely different genetic profiles. It’s not known exactly how commonplace chimerism is; outside of cows, it’s assumed to be rare, but it’s believed to exist in all species which reproduce sexually. For reasons poorly understood, a kind of reverse twinning occurs, and two zygotes, in their first few days of existence, fuse into one1. The resulting offspring carries both genetic profiles. In each organ or system, one profile will dominate, while the other occurs in a minority of cells. The primary DNA profile in one organ, however, may be the secondary profile in another. Studies of chimerical mice and sheep revealed that some individuals actually had only a single profile in some tissues, but both in others (Yu).
Chimeras can have unusual physical anomalies. True hermaphrodites—- those with both testes and ovaries—- can result from a chimera which developed from opposite-sex fraternal twins. At least two identified human chimeras currently alive were born each with a partial uterus and fallopian tube on one side of the body, and testes on the other. Pigmentation of a chimera may fall into an atypical pattern, with different parts of the body featuring dramatically different colors, or large parts of the body sporting an almost checkerboard-like pattern. Eyes, too, may be of different colors. In other cases, however, no visible evidence indicates chimerism.
Two early twenty-first century cases of chimerism sparked interest in the syndrome. Karen Keegan, a middle-aged American, became the object of study when she required a kidney transplant, and blood tests revealed that two of her three children—- who had offered themselves as donors—- did not match her genetic profile. Doctors concluded that they could not possibly be her natural children, even though she had given birth to both of them, and had been impregnated naturally. Further study revealed the second genetic profile in her other organs.
A report on Keegan published in The New England Journal of Medicine shortly thereafter found its way into a Texas court case.
Lydia Fairchild, a young mother, was fighting charges of welfare fraud, because tests had revealed that her children were not her own, as she had claimed. Since reliable individuals had witnessed their birth, and since Fairchild was pregnant once more, the judge agreed to have an official witness present at the birth of her next child. Tests once again showed that this child did not match her profile.
The doctors who had worked with Keegan became involved with the Fairchild case and have shown this second woman to be a chimera. Like Karen Keegan, she has no visible anomalies which indicate her unusual genetic make-up. The doctors who studied these cases note that, "because of the apparent rarity of tetragametic chimerism and importance of molecular techniques to confirm its presence, this condition may be underdiagnosed" (Yu). As of this writing, fewer than fifty cases of humans with this condition have been identified. However, many more could exist, and in vitro fertilization, which increases the likelihood of fraternal twinning, also increases the likelihood of chimerism (Strain).
Chimerism in humans raises interesting legal and medical questions. Lydia Fairchild almost lost custody of her children, and chimerism could also affect other cases where questions have been raised concerning parentage. In theory, a guilty individual may be excluded from a crime on the basis of DNA evidence (the plot of a CSI episode). Finally, identification of a transplant recipient as a chimera would be beneficial. Since "chimeras typically have immunological tolerance to both cell lines," such a person would have a wider range of possible donors (Yu).
1. In theory, more than two zygotes could fuse.
Sources:
I am My Own Twin. Discovery Channel. Thursday, May 19, 2005.
Annemarie Killam. "Rare Types of Twinning." BellaOnline http://www.bellaonline.com/articles/art32872.asp
Lisa Strain, John C.V. Dean, Mark Hamilton, David Bonthron. "A true hermaphrodite Chimera resulting from embryo amalgamation after in vitro fertilization." New England Journal of Medicine January 15, 1998. 338: 166-169.
Neng Yu, Margot S. Kruskall, Edmond Yunis et al. "Disputed maternity leading to identification of tetragametic chimerism." New England Journal of Medicine May 16, 2002. 346: 1445-1552.