Hello I Must Be Going: The Vanishing Twin|
5 September 2005
They walk among us. By the mid nineties, science had only smoked
out a few dozen of them worldwide but there are undoubtedly many more. You
may well know some of them. You may even be one yourself. Perhaps you're
sometimes haunted by the sense that there is someone lurking very near,
invisible and silent but sharing that odd thought, impression, or fear from
time to time. A more solid clue might be found in the skin on your back.
Sometimes you can see this plainly though they say it’s usually only
discernible under UV light.
|Photo © Ugo Bardi
I'm talking about what might be looked upon as the most extreme form of
Siamese or conjoined twinning, one in which both bodies have merged
completely into a single individual. Scientists call the perplexing result
a chimera, after the mythological beast described by Homer and others that
sported the head of a lion, the body of a goat and the rear of a dragon or snake.
What happens is that you have two fertilized egg cells that converge very
early in the gestation process. If the cells were identical twins there
would probably be no way to identify such two-in-one individuals and the
world may well be full of them. But if they're fraternal, things can get
If the two are of opposite sex you can end up with a true hermaphrodite,
though this seems to be exceedingly rare. In January of 1998 doctors in
Scotland reported the birth of a child, originally conceived through in
vitro fertilization, who ultimately tested out to present both female (XX)
and male (XY) chromosomes and corresponding equal-opportunity genitalia.
Most of the time the consequences are much more subtle. In 1953 an English woman
named Patricia McDonnell underwent some routine tests when she became
pregnant and discovered she carried both Type O and Type A blood in a ratio
of about 13 to 1. After considerable study her doctors concluded that
the minority Type A was her own and the Type O was what was left of her
Sometimes a chimera will have a left and right eye of different colors
(like Jane Seymour and Joe Pesci... hmm, do ya think?), while others — as
alluded to in the first paragraph — may display marbling or streaking
patterns on their backs, called Blaschko’s lines, which suggest an
imperfect blend of two differing complexions. Researchers call the latter
individuals mosaics. They're intrigued with the phenomenon because they
suspect certain afflictions may arise from it such as cancer, Alzheimer’s, and autism.
(Indeed, Susan Folstein of Tufts University School of Medicine in Boston
reports that about one in ten autistics show Blaschko’s lines. The
inference is that there is an antagonistic mingling of chimeric brain cells
that have trouble communicating with each other.)
Beyond all of this, surgeons and researchers can and do intentionally create
interspecific chimeras, where they combine tissues from different animal
species. Considering that about five hundred prospective transplant patients die
in the U.S. every month waiting for human donors, this can obviously be a
very good thing. Pig and cow heart valve transplants are already quite common.
Here are two links at least tangentially related to chimeras. This one deals with a rather eerie,
way-out aspect in a book by Bill Chalker; while this one explores
mammalian interspecific hybrids, some involving domestics.
© Peter Blinn 2005