On this month's Morbidlly Fascinating Page:

Chimeras: those that absorb their own twin in the womb

Chimera is pronounced

Ki-MERE-ah

(Photo gallery of Chimera animals on the bottom of this page)

WHAT IS A CHIMERA?

In Greek mythology, a chimera was a fire-breathing creature with physical traits of a lion, goat, and dragon. In human beings, a chimera is a person who has two totally different sets of DNA inside their body.

A chimera is essentially one person or animal that's made up of cells from two or more "individuals"—that is, it contains two sets of DNA, with the code to make two separate organisms all in one single body.

One way that chimeras can happen naturally in humans is that a fetus can absorb its own twin. This most often occurs with fraternal twins, if one embryo dies very early in pregnancy, and some of its cells are "absorbed" by the other twin. The remaining fetus will have two sets of cells, its own original set, plus the one from its twin. It is the same set of circumstances for animals with chimera.

Chimeras used to be thought of as rare, but once DNA testing became the norm, it is now found that it is not rare at all, especially in families prone to having twins.

In most humans, you cannot tell if they are a chimera unless you do a DNA anlysis, because it is not obvious on the outside. But with animals, it is easier to tell because they often have different eye colors or different fur colors or markings that seem to be separated by an invisible line.

WHAT ARE THE ODDS THAT YOU COULD BE A CHIMERA?

Statistically, about 8% of twins could end up with chimerism. In the case of triplets, the chance for chimerism rises up to 21%.

But can non-twins be a chimera? Yes. Vanishing Twin Syndrome can be the cause of chimerism in some of these cases. In 5% of pregnancies, multiple embryos occur which can lead to a twin birth, but not all the time. Of those, 25% of the time one fetus may fail to survive the full nine months. In this case, the embryo is either fused with the remaining fetus, or sometimes taken back up into the mom's tissues to possibly later combine with a new fetus in a later pregnancy, causing chimerism.

After examining rates of mothers with numerous embryos, it is speculated that 1 in 80 pregnancies may result in a vanishing twin pregnancy. Of the vanishing twin pregnancies, it is unknown what percentage turns a later fetus into a chimera.

See more HERE

KAREN KEEGAN

In 2002, news outlets reported the story of a woman named Karen Keegan. Two eggs fertilized by two sperm coincided in a uterus and, instead of giving rise to two sisters, they fused to form a single person: Karen Keegan. When she was 52 years old, this woman from Boston, Massachusetts, suffered very serious kidney failure, but luckily she had three children willing to donate a kidney to her. The doctors did genetic tests to see which offspring was most compatible and they got a major surprise: the test said that two of them were not her children. The reality was even more astonishing: Karen Keegan had two different DNA sequences; two genomes, depending on the cell you looked at. 

LYDIA FAIRCHILD

Probably the best-known case of chimerism is Lydia Fairchild.

Coincidently, also in 2002, Lydia Fairchild was a proud mother who faced the most unusual of challenges. She had to fight in court to prove the children born from her body were her own.

"I knew that I carried them, and I knew that I delivered them. There was no doubt in my mind," Fairchild said.

Fairchild's fight for her kids began when she was 26 years old, unemployed and applying for public assistance in Washington state. She and her children had to be tested to prove they were all related. The Department of Social Services called Fairchild and told her to come in immediately. 

The DNA test results challenged everything she knew about her family. Yes, her boyfriend was the father of the children, and, yes, they were all related, according to the DNA, except for Fairchild. She was told she wasn't the mother.

See how it turned out for Fairchild HERE

TAYLOR MUHL

Unlike the women chimeras shown above, Taylor Muhl's chimerism is obvious on the outside. As a teen and young adult, Muhl faced autoimmune challenges and continued to go undiagnosed by every doctor she saw. She was told by doctors that the physical trait on her torso was just a birthmark. Muhl was only diagnosed in 2009, after being seen by a doctor who had previous experiences in genetics and medical rarities.

See more about Taylor Muhl HERE

Go to Taylor Muhl's personal website HERE

PHOTO GALLERY OF ANIMALS THAT ARE CHIMERAS

Chimerism is not uncommon in budgies (known as half sider budgies) and are formed when two fertile yolks come together in one egg.