CHIMAERISM:
Far back in Greek Mythology stories were told of horrifying blends of many creatures. These chimaeras were seen as monstrous abominations: animals that weren’t real, that only existed as horrors in our own head.
Alongside these stories, however, existed creatures which actually were chimaeratic: they showed obvious signs of having more than one creature inside of them.
A tortoiseshell cat may contain two sets of unique DNA, resulting in the bicoloured fur.Obvious chimaeras are hermaphrodites: born with the sex organs of both genders. The common tortoiseshell cat is a strong example of chimaerism: its fur looks like two separate cats were cut up and restitched at random. This is actually fairly close to the truth: the separate patches contain different sets of genes, giving two different phenotypes.
More subtle, but still visual chimaeras can be seen in things such as a person having two radically different sides their two body, or bicoloured eyes.
So whilst we have known about chimaeras for a long time, they weren’t properly understood until we could study DNA in more detail.
Research has shown that a chimaera carries two distinct sets of DNA: the first giving rise to one of the phenotypes, the second giving rise to the other.
True chimaeras are formed from two fertilised egg cells exchanging DNA. Two eggs (recently after being fertilised) may bump into each other, and exchange cells. These cells will be accepted into the other embryo, but will still retain the DNA of the twin. As the embryo grows, part of it will be from the other twin. Note only fraternal twins can be chimaeras. Whilst identical twins can also swap cells like this they share the same DNA, so the resulting embryos still only contain a single set of DNA.
The amount exchanged may be small or large: some twins may only share parts of other skin whilst others have swapped entire organs.
Even a single child may be a chimaera. If one embryo dies in the womb, or the two completely fuse, a single child sharing both sets of DNA is born. The other way is if two sperm both fertilize an egg (the two sperm can be from a single male, since no two sperm have the same DNA). In the case of two sperm, when the egg splits one side will take after one sperm, the other half after the other one. The fusing of eggs or double insemination is what gives rise to true hermaphrodites, where one half (side) or the person is grown from a male egg, whilst the other half (side) of the person comes from a female.
An example of Chimaerism was found in a blood donor referred to as Mrs Co. who had blood of both A and O type. Subsequent testing showed she had a small amount of cells were 46/XY and were creating a small amount of A-type blood cells in her body. This is an example of a chimaera who would have gone through life without knowing of their dual-DNA except for genetic testing.
REFERENCES:
http://www.vivo.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html
http://www.springerlink.com/content/avjcdwgwm8erhpec/
http://www.newscientist.com/article/dn11464-marmosets-may-carry-their-siblings-sex-cells.html
http://jmg.bmj.com/cgi/content/abstract/31/10/816
http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1013585&blobtype=pdf
