This vacation I had a chance to watch some TV, in addition to spending time with the family, reading some books on wine and music, etc.
The most eye opening thing that I saw over vacation was some TV show on human chimerism, with at least two cases where people did *not* have any visible/physical signs of the genetic condition, but where they had two full sets of DNA in their bodies.
The term, "chimerism", comes from Greek mythology and refers to a multi-headed creature combining serpent, goat, and lion physiology (picture).
Now in human chimerism, there can be very visible markers. For example, in some reports of human chimerism (which is very rare as I understand things), people have documented checkerboard skin patterns on the chest where there is essentially a line going down the middle of the body, and where alternating squares (perhaps one inch by one inch blocks) of skin have light and dark pigmentation. To a casual observer, the checkerboard pattern may look perfectly square. In other reported cases, the person has adopted both male and female DNA. Again with a line going down the middle of the body from the head to the belly button, one half of the body may have male organs versus female. Surgery may be required to adjust the person to reconstruct the person as one sex or the other.
What blows my mind, however, is the case where the person looks and feels perfectly normal. As in turns out, it is believed that these people came to being by the fusing of two fully fertilized eggs as with Siamese twins, but because the cells did not split (say by day 4 of conception – don’t quote me on the specifics here since I’m doing off the top of my head), the embryo started to develop as one single baby with two *full* sets of DNA. Based on genetic, surgical, chemical, etc. research, it is believed that in these subset of cases that I am talking about, that while the person’s body consists of two full sets of DNA, any given organ may predominantly develop with one full set of DNA or the other (e.g., liver could have one full set of DNA from one egg, but the skin could have the full set of DNA from the other egg).
The cases caught on the TV show kind of blew my mind in terms of implications and how assumptions can be challenged. To make a long story short, two female chimeras from totally separate walks of life where shown to not have direct genetic connections to at least one of their children (by birth and eliminating possibilities of surrogate motherhood). One of the women was trying to prove that her kids were genetically hers, but DNA testing failed. Subsequent and more elaborate DNA testing showed that her kids adopted the DNA of some crazy combination of her father and her brother (this was after a court ordered that the birth of her forthcoming child be witnessed and have a forensic lab person collect blood, DNA material, etc. during childbirth – a DNA match between her and her child was negative).
DNA has often been used in courts of law to substantiate heinous crimes or to vindicate wrongly accused. DNA has often been thought of as having a one-to-one mapping with a person. Now the cardinality of the relationship may be challenged. (Note: to put some of this in perspective, however, the TV show stated that there may only be 40-60 cases [don’t quote me on number] of chimerism reported in the world, regardless of whether condition is physically visible)
Not a normal post by me by any means. But these are things that make me go …. hmmm. When I feel my (stereotypical) female traits coming through, well this discovery gives me moment to pause.
Update (2/21/06): See comments below on Siamese twins. It is likely that my reference to Siamese twins is incorrect.