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Q&A: How Could My Sleuth Recognize a Chimera?

05 Jul

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?

Susanne Shaphren
Phoenix, Arizona
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.

 
17 Comments

Posted by on July 5, 2010 in DNA, Medical Issues, Q&A

 

17 responses to “Q&A: How Could My Sleuth Recognize a Chimera?

  1. Pat Brown

    July 5, 2010 at 3:31 pm

    People have done some interesting digital chimeras. Not very useful in crime fiction, but it could be fun for fantasy or SF stories. Googles images of chimeras. Bizarre is all I can say.

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  2. Teresa Reasor

    July 5, 2010 at 8:35 pm

    Dr. Lyle:
    I have your book and found it facinating. And this condition in particular was especially intriguing.
    I also think Monozygotic twins would be intriguing too. One normal the other with his/her organs on the opposite side of their body. I have an idea for a murder mystery.
    I so enjoy your blog!
    Teresa R.

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  3. Gerrie Ferris Finger

    July 6, 2010 at 6:25 am

    Dr. Lyle,

    You’ve made it easy to understand, but I’d still have a hard time explaning, unless someone was really intently interested.

    Thanks.

    Gerrie

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  4. April Henry

    July 6, 2010 at 6:40 am

    I’ve read that sometimes chimeras have two different colored eyes, or one “hitchhiker’s” thumb.

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  5. Karen Tintori

    July 6, 2010 at 7:40 am

    Dr. Lyle —

    Might a chimeric condition be discovered by someone researching their genetic heritage (for genealogical reasons or just out of curiosity) by submitting a cheek swab to one of the DNA testing centers?

    I’ve only done the 12 marker tests. Would that suffice, or would you need to do deeper testing to discover you were a chimera?

    Could this be a way for a fictional character to make the discovery?

    Now I’m sitting here wondering how I’d feel if I found out I was a chimera. Wowser.

    KarenTintori

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    • D.P. Lyle, MD

      July 6, 2010 at 10:50 am

      Possible but only if the buccal cells were chimeric—and they could or could not be. Otherwise they would have only one DNA and since both types had to come from the parents, it shouldn’t mess up the genealogy.

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  6. Craig Faustus Buck

    July 8, 2010 at 12:44 pm

    Hi Doug,

    If a chimera’s blood was found at a crime scene, would it be obvious to the crime lab that the two different DNA patterns came from the same person because of a propensity of genetic similarities between them? Or could they be mistaken for DNA from two different people?

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    • D.P. Lyle, MD

      July 9, 2010 at 12:23 pm

      Maybe. In some cases the two types of DNA are seen throughout the body and in other cases the two types show up in some tissues and only type in other tissues. For example, the blood could show one type and the liver the other or they both could show both types or the liver could show both types and the blood only one. It can be very confusing in some cases and in others go unnoticed.

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  7. Shaena

    April 10, 2011 at 2:07 pm

    I just found out that I am a chimera (vanishing twin kind) because I donated blood. I have always been B+ when tested when I was young, my mother’s blood type, and she donated a lot of blood to me when I had heart surgery when I was young since we matched. Recently, I gave blood and was typed O. My father is type O. I have two extra ribs, but not other indication of being a chimera. From what I have read I am a blood chimera, but the extra ribs suggest I could have two sets of dna in different tissues throughout my body. Not sure if this is helpful, as it is pretty belated.

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    • April Henry

      April 10, 2011 at 6:05 pm

      I’m still following this thread and I think that’s really cool, Shaena. Or does it feel strange to think that you are a mix of two different people?

      April

      aprilhenrymysteries@yahoo.com

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  8. Helen

    June 7, 2011 at 1:51 pm

    I have one hitchhiker’s thumb. Is that always a sign of chimera? I feel really weird about it.

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    • D.P. Lyle, MD

      June 7, 2011 at 1:52 pm

      No that does not mean you are chimeric.

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      • April Henry

        June 7, 2011 at 2:22 pm

        But is it possible that she is?

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      • D.P. Lyle, MD

        June 7, 2011 at 4:05 pm

        Yes. Most people with the disorder don’t even know it.

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  9. Anne

    April 27, 2012 at 2:14 pm

    There can be a number of indicators of chimerism depending on the genetics of the individual cells. For example Blaschko’s (sp?) lines. This is differences in skin pigmentation making patterns akin to zebra’s stripes. In people with a lateral distribution between the two cell lines you can find differences such as a hitchhiker’s thumb or hanging lobe (or any number of quirky genetic traits that may not impact one’s health) on the ear on one side only.
    Another is detection of mixed blood groups (blood types) most European and North American countries now screen for mixed blood groups (Having the presence of the two blood types in your system. Mixed blood groups happen in those with microchimerism as well as tetragametic chimerism (the type you are thinking of), but it can start a line of inquiry.

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