21st April 2011 Chimeras

A chimera is an animal which has two distinct sets of genetically different cells arising from different fertilised eggs. If these different cells arise from one fertilised egg, it's called mosaicism.

A while back, in an episode of CSI, a man was found to have two different sets of DNA. Is this just fiction or does it really happen in humans?

Formation of Chimeras and Mosaics

The ways in which chimeras can form are as follows.

• Fraternal twin embryos fuse together to become a single embryo, so many cell types could be mixed.
• Fraternal twins share a blood supply and their blood mixes. Blood stem cells from one settles in the bone marrow of the other so that blood of the other twin can continue to form. This type of chimera is blood only.   [4]

Mosaicism occurs when there is an error in the way the cells divide in the embryo. Both the original and the mutated cells split and, if this occurs early enough, cells end up with a different genotype.[4]

Examples

A real example of this happened in 2002 when an American lady, Lydia Fairchild was tested and found to not be the mother of her children. On further testing of a sample taken from a different part of her body, her DNA matched the children's.

An earlier record, from 1998, is of a lady who needed a kidney transplant. Her bone marrow showed just one set of DNA yet other cells showed two separate sets.  She had all four sets of chromosomes from her parents. All her cells were XX so most likely two eggs, fertilised with X chromosomes, merged.[8]

It is possible that XY and XX embryos merging form what was called a hermaphrodite but is now referred to as intersex in medicine.

Problems with Forensics

Identifying fathers, body parts from a plane crash, or blood from a crime scene might be wrong if it is unknown that someone is a chimera.

The probability of chimeras is unknown, but with the event of IVF, it is more likely as twins are required for it to occur.

Sources

1. Wikipedia: intersex
2.  Wikipedia: hermaphrodite
3.  Wikipedia: Lydia Fairchild
4.  Stanford University: Understanding Genetics -Ask a Geneticist 
5. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Mosaics.html#TetragameticHuman 
6. Flatrock: dual identities
7. Wikipedia: Blood type history 
8. NPR podcast on chimerism
9. http://blogs.discovermagazine.com/notrocketscience/2011/04/21/the-many-yous-in-you-–-what-lydia-fairchild-has-in-common-with-a-sponge-and-an-anemone/