Are you a chimera?

Thousands of years ago, the Greek poet ­Homer wrote about a specific monster with the chest of a lion, the tail of a snake, the midsection of a goat and heads of all three animals. While this is only mythology, and an exaggerated version at that, there are many natural-borne chimeras around today that we may not even know about.

What is a chimera?

From wiki: a chimera is a single organism (usually an animal) that is composed of two or more different populations of genetically distinct cells that originated from different zygotes. Chimeras are formed from four parent cells (two fertilized eggs or early embryos fused together). Each population of cells keeps its own character, resulting in an organism of mixed tissues. Usually, the condition is inherited, but may also be acquired through the infusion of allogeneic hematopoeitic cells (this happens through transplantation or transfusion). The likelihood of offspring being a chimera is increased if it is created via in vitro fertilization.

There is also a form of congenital chimerism (tetragmetic chimerism). In this way, chimerism occurs through the fertilization of two separate ova by two sperm. The two usually then fuse together at the blastocyst or zygote stage. What results is an organism with intermingled DNA. As the organism develops, it can come to possess organs that have different sets of DNA (i.e. it may have the liver composed of one DNA and a kidney of another. It may even have two different blood types). In CSI (the original), this was a particular key plot in "Bloodlines".

The difference in phenotypes may be subtle (e.g., having one eye a different colour from the other, etc.) or completely undetectable. Chimeras may also show, under a certain spectrum of UV light, distinctive marks on the back resembling that of arrow points pointing downwards from the shoulders down to the lower back in what is called Blaschko's lines.

Blaschko lines form from the fact that chimeras start out with two different cells, each with different DNA (and therefore different instructions). The skin of the person is therefore made up of two different sets of instructions on how to colour the skin. The Blaschko's lines result from the fact that some of a chimera's skin cells say to make darker skin and some say to make lighter.

When there is a big difference between the two DNA's instructions on how dark to make the skin, then you get obvious Blaschko's lines. If the differences are subtle, then you may not be able to see the pattern without the aid of an UV light.

While not as rare as once believed, chimeras may be identified by finding two different populations of red cells, or if the zygotes were of opposite sex, either ambiguous genitalia or hermaphroditism (alone or in combination).

In 2003, scientists had begun to blur the lines of chimerism-producing animal-human hybrids-when Chinese scientists at Shanghai Second Medical University successfully fused human cells with rabbit eggs. While the embryos were successful, after several days of being allowed to develop in a laboratory dish, they were destroyed so that the stem cells could be harvested. And in 2004, pigs were created with human blood.

Why?

Because scientists believe that the more human-like the animal, the better research model it makes for testing drugs or possibly growing "spare parts," such as livers, to transplant into humans.

What do you think about chimeras?