This is a natural outcome of our
improving knowledge regarding DNA flexibility itself which is no longer seen as
a fixed system but one that changes throughout life.
Through items like this it is
possible to capture the ongoing emergence of a vastly more dynamic model for
human DNA.
It is a revolution in
understanding that remains invisible to the popular consciousness.
Identical Twins Are Genetically Different, Research Suggests
By Tia Ghose, Sat, 10 Nov,
2012
The findings, presented Friday (Nov. 9) here at the American Society of
Human Genetics meeting, may partly explain why one twin gets cancer while
another stays healthy. The study also suggests that these genetic changes are
surprisingly common.
"It's not as rare as people previously expected," said study
presenter Rui Li, an epidemiologist at McGill University
While past studies have looked at genetic changes, or mutations, in
sperm and eggs, which can be passed on to offspring, very few studies have
looked at somatic
mutations.
These mutations, also called copy errors, can occur early in fetal
development, but because they aren't in the sex cells (the X or Y chromosomes)
of the fetus, they can't be passed on.
Other studies have shown that chemical modifications, or epigenetic
effects, can change which genes are expressed over the years, one factor
that renders twins not completely identical. Still, other work has shown
that identical
twins can have different gene mutations, but this study didn't
determine how often they occur.
To find out how often these mutations occur in early development, Li
and her colleagues studied the genomes of 92 pairs of identical twins and
searched hundreds of thousands of sites in their genomes for differences
between twins in base pairs, which are represented by letters that make
up DNA.
For instance, one twin may carry an A at one point while another carries a C.
The researchers could only detect differences that would occur very early in
fetal development and would show up in most cells in the body.
They then calculated the frequency with which these mutations occurred.
Only two sets of twins had such mutations, which translates to a DNA change
occurring once for every 10 million to 10 billion bases that are copied every
time a cell divides. While that may seem like a high accuracy rate, cells in
the body divide trillions of times. So that would mean an average twin pair
carries 359 genetic differences that occurred early in development.
One limitation of the study is that they could only estimate the
mutation rate based on blood cells, but some cells in the body divide much more
frequently and so may rack up many more mutations. Other cells, like brain
cells, don't regenerate much and would probably remain stable.
"Our DNA samples came from blood samples," Li told
LiveScience. "You need to define different rates in different
tissues."
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