http://www.nature.com/nsu/020429/020429-13.html - snippets from an
interesting longer article.

The frequencies of chimaerism and mosaicism are unknown, but doctors
might benefit from a better understanding of both conditions. In recent
years, tantalizing hints have emerged that pockets of genetically
mismatched cells may contribute to conditions as common as infertility,
autism and Alzheimer's disease. "I think mosaicism has been neglected as
an underlying cause of disease," says Huntington Potter, who works on
the genetics of Alzheimer's at the University of South Florida in Tampa.

And if chimaeras and mosaics are more common than we realize, they will
complicate future efforts to tailor drug treatments to people's
individual genetic constitutions. Two genetically different tissues in
one body might produce an unpredictable response to a drug, speculates
Roland Wolf, who studies pharmacogenetics at the University of Dundee,
UK. "It's completely unknown," he says.

The twin within
Human chimaerism first came to light with the advent of blood typing -
some people, it emerged, have more than one blood group. Most are 'blood
chimaeras', non-identical twins who shared a blood supply in the womb.
Those who were not born a twin are thought to be pumping around the
remnants of a sibling that died early in gestation and was spontaneously
aborted. One British woman, for instance, was unaware that she once had
a twin until routine blood tests during her pregnancy in the early 1980s
revealed a population of chromosomally male blood cells2.

Twin embryos often share a blood supply in the placenta, allowing blood
stem cells to pass from one embryo and settle in the bone marrow of the
other, seeding a lasting source of blood. As a result, as many as 8% of
non-identical twin pairs have chimaeric blood3. And given that most
multiple conceptions that result in live births involve the loss of one
twin early in pregnancy4, there may also be significant numbers of blood
chimaeras among single births.

Even more people have 'microchimaerism', carrying smaller numbers of
foreign blood cells that may, for instance, have passed between mother
and fetus across the placenta, or persist from a blood transfusion. Some
researchers argue that the presence of foreign white blood cells might
help to explain autoimmune diseases, in which the immune system turns on
the body's own tissues5.

Hard to spot

True chimaeras, including both of those identified by Bonthron's team,
generally only come to light if they contain male and female cells, when
they can cause hermaphroditism or a mismatch between a person's sexual
organs and their chromosomal sex as revealed by a blood test. So might
the condition be more common than we realize? "I'm convinced that on the
streets of London and Hamburg there are many undetected chimaeras," says
Rudolf Happle, a dermatologist at the University of Marburg, Germany,
who has long been fascinated by mosaicism and chimaerism.

Mosaicism causes these unusual pigmented patterns called Blaschko's
lines. <see picture>
© R. Hepple

The rise of in vitro fertilization (IVF) is almost certainly bringing
more chimaeras into the world. To improve success rates, two or more
embryos are placed in the uterus, which explains why women undergoing
IVF have up to 25% more twin pregnancies than usual. More twins means
more chimaeras, says Bonthron, who notes that the British hermaphrodite
boy was an IVF baby1.

Mosaicism is more common than chimaerism and is also better studied.
Human mosaics arise when a mistake during cell division in the early
embryo stops the correct number of chromosomes segregating to each cell,
or creates a mutation in a single gene. If this happens in one of the
first few cell divisions after fertilization, a large proportion of
cells will inherit the defect.

Patchy diseases, in which only regions of tissue are affected, might be
caused by mosaicism. Another telltale sign of the condition is a
characteristic variation in skin pigmentation that causes patterns
called Blaschko's lines, including V-shaped streaks on the back that are
sometimes only visible under ultraviolet light.

Other researchers suspect that mosaicism could be involved in more
common diseases. Wendy Robinson of the University of British Columbia in
Vancouver, for instance, is intrigued by the observation that the
placenta becomes mosaic in about 2% of pregnancies. Often, these mosaic
placentas contain patches of cells that have an extra chromosome - a
condition called trisomy. Because both fetus and placenta develop from
the same cells, Robinson wonders if many fetuses contain undetected
patches of trisomic tissues that persist into adult life. "People could
have little pockets of trisomic cells sitting around inside them that
later lead to disease," she suggests. Intriguingly, her team has found
that some women who experience recurrent miscarriage carry trisomic
cells7.