Tough Enough For Two

At Roger’s 100-day post-transplant bone marrow biopsy, his DNA was 98% identical to his donor’s. A few days ago, we were speculating about whether he has any of his own DNA left by this time or if he is now 100% identical to his brother.  This is what passes for casual conversation at our house now.  Weird, huh?  The terms chimera (pronounced: ki-mira) and chimerism (pronounced: ki-merism) came up and Roger thought it was a subject interesting enough to blog about, so here we go.

The original chimera was a creature in Greek mythology, the offspring of two gods, composed of parts of two or more animals.  According to The Iliad (not that I can claim to have read it) it is "a thing of immortal make, not human, lion-fronted and snake behind, a goat in the middle, and snorting out the breath of the terrible flame of bright fire."   Something like this:

Fake.

The term chimera came to be synonymous with “hard to believe.”  Thanks to Photoshop, you see all sorts of funny examples of chimerism on the internet. They are clever compositions, but are just as hard to believe.

Fake.

In real life, chimerism occurs when an animal has two or more distinct populations of genetically different cells.  One animal, or one person can have two sets of DNA, but it does have to be within the same species.  A combination parrot/blowfish could never happen except in Photoshop. Chimerism is a condition that can be inherited or a mutation developed later in life, and it is not as rare as you might think.   Most chimerae or “mosaics” go through life without realizing they are chimeras.  The differences may be obvious (e.g., eyes of different colors, different hair growth on opposite sides of the body, patchy colored hair or skin tones) or the differences may be so slight as to be undetectable.  Affected persons are sometimes identified by the discovery that they have two populations of red blood cells.

Real.

Tortoiseshell cats are not a specific breed.  They are chimeras, resulting from the fusion of the genes for two different hair colors when the cat is developing as an embryo.

Real.

A bone marrow transplant turns the recipient into a chimera.  Roger’s new bone marrow is busy manufacturing new blood cells with his brother’s DNA type, so the DNA in his blood is different than that in the rest of his tissues.

Real.

In theory, this could complicate a criminal investigation.  “DNA typing” is based on the concept that every cell in the body contains identical DNA, but as we’ve just discovered, it's not reliable in all cases.  Did you see that episode of  Law & Order?  I offer a simple solution. Neither Roger nor his brother should go around committing crimes.  Ergo, no problem.

Chimera – offspring of the gods, it’s a beast to be reckoned with, tough enough for two.
--Candi