Conception is normally a pretty straightforward process: One lucky sperm wiggles its way into one lucky egg. In the case of fraternal twins, each sperm-and-egg pair is just one half of a double date. The resulting fertilized eggs trundle along and begin dividing separately, eventually becoming two separate fetuses. But sometimes, one egg engulfs the other, effectively “eating” its twin.
This isn't as messy as it sounds. At that stage in development, we’re all just smooshy single cells. The newly fattened twin-eating cell goes about its business, dividing and dividing, growing and growing, until, many months later, it becomes a person (or a lion, or a rabbit, or a weasel, depending on its parents).
The scientific term is tetragametic chimerism, although the origin of that word is hardly scientific. The ancient Greek Chimera was a mythical monster with the body and fire-breathing head of a lion, a goat head rising from its back, and a snake tail. Over time, the word chimera came to mean any kind of combo-pack animal—including, eventually, people with two sets of DNA.
Because, young as those twin cells are when they become one, each cell has its own set of DNA. After the Big Meal, both sets come to reside in a single cell, which eventually becomes a single animal.
People (and animals) with chimerism may be visibly different. They may have one brown eye and one blue, or one hitchhiker’s thumb and one straight one. Their skin may, almost imperceptibly, be a swirl of two different colors. Chimeric animals can sport a two-tone pattern split right down the middle. That appears to be the case for Twinzy, pictured above checking out his reflection in a mirror, a budgie who lives at Rudy's Pet Supply & Feed in Oklahoma City.
Most of the time, though, chimeras look just like everybody else. That was the case for Lydia Fairchild. As a pregnant, single mother of two applying for welfare benefits, in 2002 Fairchild and her children had to undergo paternity testing. The results confirmed that the father of Fairchild’s children was, in fact, their father. But, according to the DNA tests, Fairchild wasn't their mother.
The state threatened to take her children away. Fairchild was accused of welfare fraud and illegal surrogacy. The DNA evidence against her seemed airtight, and even her own parents began to doubt her story.
Then her lawyer saw a story in the New England Journal of Medicine that seemed startlingly familiar. A woman named Karen Keegan had been in need of a kidney transplant. To identify a possible donor, Keegan and her family members had their DNA tested. The results came back: Keegan could not be the mother of two of her three sons.
Eventually, researchers at the National Institutes of Health began testing DNA from different parts of Keegan’s body. The pieces fell into place: Keegan was a chimera. After the researchers published their findings, Lydia Fairchild’s lawyer took their findings to the judge, who granted Fairchild official custody of her own children. After the case was dismissed, scientists determined that Fairchild did, indeed, have two sets of DNA.
So it turns out chimerism isn’t all fun and games (and twin-eating). The genetic, physical, and social consequences may be nonexistent. They could be super-cool and make you look like a badass. Or they could be life changing.