Some time ago, I did a post about fingerprinting and personal identification. Now while fingerprinting has been around for more than 150 years, a new breakthrough in personal identification happened much more recently – on this day in 1984, DNA fingerprinting was discovered in Leicester, England.
The man who discovered this revolutionary new technique, was Sir Alex Jeffreys of the University of Leicester. He was the first to realised that each person’s DNA has a unique pattern, almost like a bar code, and that this could be used as a biological identification method. The technique has, over the past 25+ years, proved an invaluable tool in forensics, crime investigations and identification of genetic relationships.
Of course no technique is perfect, and in the case of DNA fingerprinting there are also rare occasions where the technique cannot be used. Identical twins, for example, have matching DNA, and so their DNA fingerprints are the same. A much rarer, and much stranger, occurrence is when a single person has more than one DNA fingerprint.
Strange as this may seem, having a single person with two distinct genetic identities is possible. This condition is known as chimerism, named after the chimera, a mythological creature with features from more than one distinct animal, for example a lion’s head and a serpent’s tail.
A human chimera is formed during pregnancy. Normally the male gamete (sperm) fuses with the female gamete (ova) to form a zygote, the cell that becomes the embryo. This embryo has a new genetic identity, formed from a unique combination of the DNA of the mother and the father. On rare occasions, two male gametes will fuse with two female gametes, to form two zygotes which develop into two (non-identical) twin embryos. These embryos will each have a different, unique new DNA fingerprint, inherited from the father and mother.
In extremely rare cases, these two embryos can fuse, growing into a single child, but formed from four gametes, and thus having two distinct sets of DNA. The chimera child can grow up without anyone ever knowing about his double identity, but may in fact have different organs or body parts that have completely different genetic fingerprints. Even when a DNA identity test is done on a chimera, DNA will usually only be taken from a single source, such as a blood sample or cheek swab, and the second identity may never be known.
Chimerism may, in rare occasions, visibly manifest itself, for example with people having both male and female reproductive organs, or two different colour eyes. (However, different eye colours, or heterochromia, can have different causes, and is, as far as I know, not necessarily an indication of chimerism.)
The most famous example of a chimera confounding DNA profiling came from a case in 2003, when a mother of three were told, after DNA tests were done on her and her three children, that two of her three sons were not hers, even though she maintained that she had conceived them with her husband, and delivered them naturally.
After more extensive testing, it was discovered that she was a chimera, and that the two sons thought not to be hers did in fact match her ‘second identity’.
Definitely a case where truth is, in fact, stranger than fiction.