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May 30, 2018 5 mins

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 episode of BrainStuff.

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Episode Transcript

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Speaker 1 (00:02):
Welcome to brain Stuff from How Stuff Works, Hey, brain Stuff,
Lauren Vogel bam here. When 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

(00:23):
most of her life. Mole says, everything on the left
side of my body is slightly larger than the right side.
I have a double tooth in the left side of
my mouth, and many sensitivities and allergies to foods, medications, supplements, jewelry,
and insect bites. While Mall 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

(00:45):
genetic code of her twin, a sister Moll had absorbed
while still in the womb. Mall is a kind mira,
someone who carries two distinct sets of DNA, each with
the genetic code to make a completely separate person. The
difference in in pigmentation on Mall's abdomen. These sized discrepancies
between her left and right sides, and a host of

(01:05):
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, 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.
Mole's condition. Tetra Amedic chimerism is the most rare of

(01:28):
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 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

(01:48):
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, 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.

(02:10):
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 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

(02:31):
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 carry blood
cells that are identical to their donor for the rest
of their lives, in addition to their own genetically distinct
blood cells. Blood transfusion, however, only produces a temporary chimera condition.

(02:51):
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 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

(03:14):
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, 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 tetragamtic and micro may someday challenge

(03:36):
how medicine, forensics, and even the legal system contend with DNA.
DNA testing, which is done by collecting small amounts of
humans aliva, semen, hair, bone, blood, or skin tissue, relies
on the notion that every cell in an individual body
contains identical DNA, and that each person has different DNA
Cimerism challenges the notion of a one DNA to one

(03:59):
person's 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, 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

(04:19):
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.
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

(04:40):
the end, investigators used a blood sample and cheek swab
to make a DNA match from the crime scene and
charged the correct brother with the crime. Today's episode was
written by Laurie L. Dove and produced by Tyler Klang.
For more on this and lots of other multi natured topic,
visit our home planet, how Stuff Works dot com. M

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