http://en.wikipedia.org/wiki/Chimera_(animal)Chimerism may occur naturally during pregnancy, when two non-identical twins (even of different sexes) combine in the womb at a very early stage of development, such as that of the blastocyst, to form a single organism. Such an organism is called a tetragametic chimera as it is formed from four gametes — two eggs and two sperm.
As the organism develops, the resulting chimera can come to possesses organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosomes and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, though it is considered extremely rare, but since it can only be detected through DNA testing, which itself is currently rare, it may be more common than currently believed. As of 2003, there were about 30 human cases in the literature, according to New Scientist.
DHC [Discovery Health Channel] ran a special called "I am my own twin", which is about two women who are both chimeras. One of these nearly lost her two children through the courts because she couldn't prove that she was the mother - the DNA didn't match. These women each have two sets of DNA, with one being most dominant and found in most parts of the body, and the other being much more difficult to find, and in one case, only found in certain organs. Only after extensive testing was the other DNA located and identified; finally proving that these women are the mothers of their children as they claimed. It is a fascinating story that ends by questioning just how rare this really may be.
One of the most unusual aspects of their stories is that they are otherwise completely normal. There is no outward clue that either one has this condition. Photographs of two other chimeras were shown. One was a baby who, between the belly button and pubic region, was dark and mostly female on one side, and white and mostly male on the other, with a distinct line right down the middle. The other chimera shown actually had a black and white checker board pattern on the belly and chest!
Here is another link that popped up to a New Scientist article about this.
My schedule shows that this will run again:
Sun 8/14 7:00 PM DHC
Sun 8/21 5:00 PM TLC