identical--almost

Random academic trivia of the night. (Well actually morning, and nearly sunrise. But whatever.)

Identical twins share exactly the same DNA profile because they formed from the same zygote--a fertilized egg, from which the technical term 'monozygotic', meaning one zygote--and thus contain the same genetic material from the same sperm and ovum. So DNA tests are useless because it's impossible to tell which twin contributed the test sample. Bizarrely, it is possible to differentiate between the two by a much less technically advanced method: fingerprinting. Identical twins often have very slight variations in the patterns of their fingerprints.

There also exists a rare phenomenon called 'chimerism' which is kind of like the reverse of an identical twin. Instead of a zygote splitting and making two people sharing one profile, chimerism is when two different zygotes (which would normally develop together as fraternal twins) fuse together and produce one person who has two genetic profiles. In 2002 a Washington woman named Lydia Fairchild was not only denied public assistance but actually temporarily lost custody of her children when the standard DNA test required to apply for assistance appeared to reveal she was not their biological mother. (The test did show she was related to them--it would have to, since Fairchild's two genetic profiles still came from exactly the same parents, just like siblings.) The matter became so heated that, when she discovered she was pregnant a third time, she invited court officers and state attorneys to witness the birth and take DNA samples immediately. The result was the same, but still they refused to believe she was acting as an illegal surrogate and intended to charge her with welfare fraud; it wasn't until they became aware of a much earlier case in Boston in which another woman, Karen Keegan, was accused of not being the mother of her own children due to the dual genes of chimerism.

The reason for this is because the fusing of zygotes creates certain regions or organs to develop from one source and other parts from the other source--so that, in these two cases, the blood and saliva tested were formed with one set of genes and the other, related but still not identical, genes formed their reproductive organs. In Keegan's case, her thyroid also contained the second mystery genes.

Chimerism is extaordinarily rare--just a small handful of cases have been recorded in the last seventy years--but it does raise an unlikely but alarming possibility. A rapist, for example, might possibly possess two different genetic profiles in such a way that his semen would appear to come from a different person than the contributor of the standard cheek-scraping or blood sample used in conventional DNA tests.

Weird, huh?