In general, each individual person has DNA that is specific to them. A person who is an identical twin carries the same DNA as his or her twin. It is also possible for a person to be a chimera – who carries more than one type of DNA. One of your relatives could be a chimera without even knowing it.
What is a chimera? It is an organism or tissue that contains at least two different sets of DNA, most often originating from the fusion of as many different zygotes (fertilized eggs). In “plain English”, a chimera is a person, or animal, that has two different sets of DNA in their body.
How does that extra set of DNA get there? It happens when an organism absorbs its twin while both are in the womb. This phenomena is sometimes called vanishing twin syndrome. According to Baby Centre, out of all live births that have been conceived naturally, one in 80 will be a twin birth. Sometimes, very early on in a pregnancy, one embryo simply stops growing and is absorbed into the gestational sac.
The term “chimera” comes from Greek mythology. It referred to a fire-breathing monster that was part lion, part goat, and part dragon. In reality, a chimera doesn’t look like a monster. You may have seen photos on the internet of a cat with a face that is half tabby and half black. That cat is a chimera.
Humans who happen to be chimeras typically are unaware of it. There is the case of a woman referred to as “Mrs. McK” who did not know she was a chimera. It was discovered after she donated blood. The lab discovered that the blood she donated was of two different types. Usually, a person has only one blood type.
Later, it was revealed that “Mrs. McK” was a twin. Her brother died when they both were three years old (about 30 years before “Mrs. McK” donated blood.) Some of her brother’s blood infused into her body during gestation, and it continued to circulate.
More recently, a 34 year old American man was discovered to be a chimera. He donated sperm at a clinic. A couple that was having difficulty conceiving used that donated sperm and a child was born. The baby was healthy, but somehow, had a blood type that did not match that of his mother or of his biological father.
At first, the couple believed that the fertility clinic had made a mistake. They had the man who donated the sperm take a paternity test. The test showed that he was the uncle of the baby. His twin (who was absorbed before birth) was the actual father of the baby.
Image by Flip Maljković on Flickr.
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