BrainStuff Classics: How Can You Be Your Own Twin?

In a genetic condition called chimerism, DNA from someone else (an unborn twin, an organ donor, or even your own baby) is at work in your body. Learn how it works in this classic episode of BrainStuff, based on this article: https://science.howstuffworks.com/life/genetic/chimerism-be-own-twin.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff Lauren Vogelbaum here with a classic brain Stuff episode. Today,
we're looking at a number of conditions in which different
cells in a person's body might be operating using a
different person's DNA, including one in which you can be
your own twin. Hey brain Stuff, Lauren vogel bam Here.

(00:26):
Gwen Taylor Mall, a California based singer, asked a doctor
to investigate the birthmark that stretched across the whole left
side of her torso. She wondered what role the ruddy
patch might play, if any, in a series of seemingly
disparate health conditions she had experienced for most of her life.
Mole says, everything on the left side of my body
is slightly larger than the right side. I have a

(00:47):
double tooth in the left side of my mouth, and
many sensitivities and allergies to foods, medications, supplements, jewelry, and
insect bites. While Mole says she hoped for answers, she
was shocked by what the doctor told her. Her birthmark
wasn't a birthmark at all. She actually carried the genetic
code of her twin a sister Mall had absorbed while
still in the womb. Mal is a chimera, someone who

(01:11):
carries two distinct sets of DNA, each with the genetic
code to make a completely separate person. The difference in
skin pigmentation on Mall's abdomen, these sized discrepancies between her
left and right sides, and a host of autoimmune symptoms
are caused by two warring sets of DNA. In Greek mythology,
a chimera is a fire breathing female monster that's part lion,

(01:33):
part goat, and part dragon. As remarkable as this creature sounds,
real life chimeras are perhaps even more astounding and probably
more common than we realize. Mall's condition, tetracomedic chimerism is
the most rare of all types of chimerism. It occurs
when two separate eggs or ova, are each fertilized by
a separate sperm. Then the fertilized eggs, called zygoats, fused

(01:55):
together to create a single organism with two genetically distinct
types of DNA. As the embryo cells multiply and grow,
so do the two different types of genetic material. The
result is a baby who is born a chimera. Often,
baby is born with chimerism have patchy skin or eye pigmentation,
and sometimes they carry two distinct types of red blood cells. Occasionally,

(02:16):
chimeras are born with ambiguous genitalia or possessing attributes of
both male and female sex organs. These symptoms can occur
not only in humans, but in other types of mammals,
including mice. Chimerism was once believed to be extremely rare,
but such conditions are commoner than we realized, as Linda Randolph,
m d. Told The New York Times. Randolph is a

(02:37):
pediatrician at Children's Hospital in Los Angeles, an author of
a chimerism review published in the American Journal of Medical Genetics.
In addition to genetic chimerism, the condition is commonly caused
when another genetic code is introduced to the body by
organ or tissue transplants, as well as by blood transfusions.
For example, when someone undergoes a bone marrow transplant, they'll

(02:58):
carry blood cells that are identical to their donor for
the rest of their lives, in addition to their own
genetically distinct blood cells. A blood transfusion, however, only produces
a temporary chimera. Condition, The donor blood cells will live
about a hundred and fifteen days in the host's body
and will eventually be completely replaced by red blood cells
carrying their original genetic code. These types of chimerism are

(03:20):
commonly known as microchimerism, and one of the most frequent
causes is pregnancy. Trace fetal stem cells often remain in
a mother long after she has given birth. Some of
these stem cells have even been found to reside in
women's brains. Likewise, a pregnant woman's cells can cross the
placenta and become part of their child's liver, heart, thymus, gland,

(03:40):
and bloodstream. Many people may not ever suspect that microchimerism
is part and parcel of their existence, but it's probably
more frequent than we realize. In fact, chimerism, both tetragametic
and micro may someday challenge how medicine, forensics, and even
the legal system contend with DNA. DNA testing, which is
done by collecting small amounts of human saliva, semen, hair, bone, blood,

(04:04):
or skin tissue, relies on the notion that every cell
in an individual body contains identical DNA, and that each
person has different DNA chimerism challenges the notion of a
one DNA to one person system. In one complex assault case,
for example, evidence at the scene matched a DNA blood
sample from a criminal already profiled in a law enforcement database. However,

(04:28):
this person was in jail when the assault occurred. To
make matters even more confusing, the DNA blood sample also
matched the DNA profile of another person who could also
have been implicated in the crime. After investigating, it was
determined that the two men were brothers. The man in
jail had previously received a bone marrow transplant from his brother.

(04:48):
This means that the jailed man was a chimera. His
blood DNA profile matched that of his brother's blood DNA,
while his saliva DNA profile was distinctly his own. In
the end, investigators used a blood sample and cheek swab
to make a d n A match from the crime
scene and charged the correct brother with the crime. Today's

(05:11):
episode is based on the article calimerism you Can be
your Own twin on how stuff Works dot Com, written
by Laur L. Dove. Brain Stuff is production of I
Heart Radio in partnership with how stuff works dot Com,
and it is produced by Tyler Clang four more podcasts
from my heart Radio. Visit the i heart Radio app,
Apple podcasts, or wherever you listen to your favorite shows.