This will be the first post of a new series I’ll add to the blog from time to time. I get cool questions from writers all the time so I thought it might be fun to post some of the most interesting ones here.
Back in 2004, I received a question about chimerism from Susanna Shaphren. I included it in my book FORENSICS & FICTION. I thought it might help readers understand this fascinating medical condition a little better so here it is:
Q: My question is about chimeras, individuals whose body is made up of two genetically different lines of cells. The case I read about involved a woman who underwent tests to determine if one of her sons was suitable as a kidney donor for her. Testing determined that two of her sons were NOT her biological children. Their DNA came from the mother’s twin, who instead of surviving to be born was somehow absorbed into the birth mother’s body.
I would love to use this as a springboard for fiction, but I need some help to be sure I’m on at least semisolid ground. Unless the character with this condition needed an organ transplant, would there be any other possible way for the condition to be diagnosed?
Author of “Arrangements,” in Mystery Writers of America Presents Shoe Business Is Murder and “The Best of Friends,” in Sex, Lies, & Private Eyes
A: When an egg and a sperm join to make a fertilized egg, the genetic makeup of the offspring is set at that moment. Normally the cell will divide into two, and those into four, and those into eight, and so on. At some point in the growth of the zygote the cells begin to specialize or what we call differentiate. Some will become brain tissue, others blood cells, and others muscle cells.
In fraternal twins two eggs are fertilized by two sperm and this process occurs in parallel, and two entirely distinct individuals result. In identical twins the original fertilized cell (egg) divides into two cells, but these two drift apart, and then each proceeds along the growth path in tandem. This creates two individual with identical genetics. After all, they started from the same cell and thus from the same egg and sperm. So far so good.
In chimeras fraternal twins are formed (two eggs and two sperm, and two genetically different individuals) but these two original cells (fertilized eggs) stick together. As growth takes place the developing zygote is composed of two distinctively different cell types with two distinctively different genetic make-ups. As these cells begin to specialize, some organs and tissues may come from one type of cell and some from the other, and still others may develop with a mixture of cell types. This leads to a chimera, where various body tissues (liver, blood, skin, heart, brain) may have one or the other or both of the two original DNA profiles. This can lead to confusion in any testing dependent upon DNA typing.
Chimeras may appear normal or may display certain mosaic patterns, particularly unusual pigmentation patterns, on their skin. This is merely an expression of their two genetic types. A mosaic in art is something made up of different-appearing distinct pieces. The same holds true here, since the cells of the person contain separate and distinctive DNA patterns.
If the person were normal in appearance, the only time a chimeric condition would be diagnosed would be if DNA testing were undertaken. This is done in organ transplantation, paternity testing, and in criminal situations, to name a few circumstances. Otherwise the person may never know of his condition.
If your character displayed an odd mosaic skin pattern, your sleuth could see this and suspect that the person was chimeric. These skin patterns can be almost anything, even areas of a distinct checkerboard pattern.