Identical twins have the same DNA (almost)
 April, some of
my friends here have been arguing about a DNA test of identical twins. One says
they should be identical; the other says they are different.
T.M.H., Webster,N.Y.
DNA patterns from four sets of twins-which are identical? Photo courtesy of
Robert J. Huskey, U of Virginia
Your first friend wins — mostly. Identical twins have almost identical DNA.
We used to think identical twins had identical DNA, because they come from the
same fertilized egg, which had only one complete set of DNA in the first place.
That fertilized egg and its single set of DNA split to form twin embryos — each
with its own set of DNA, identical to the other. But we admitted one small
quibble: the DNA sets will be identical, unless one fertilized egg's DNA
mutates. Such mutations, however, are normally so tiny that DNA analysis can't
detect them.
However, in February 2008, geneticist
Carl
Bruder et al reported results of a study of nineteen identical twins.
The team found relatively common, but small, differences in the DNA of these
identical twins. For some twins, one twin had a different number of copies
of a given gene than his twin. This condition is called 'copy number
variant' or CNV.
People normally have two copies of each gene, one from each parent. But
sometimes things go awry, and the two-copy rule fails. A person may be
missing a gene, or have three or four copies of a given gene. But, however
faulty the genes identical twins inherit from their parents, each twin gets the
identical genome as the other twin, because the fertilized egg (and its DNA)
splits to form twin embryos.
But how, then, do identical twins end up with a different gene-copy count?
On Day 1, the initial split of the fertilized egg forms the first two cells
(each one the start of a twin), and each initial cell does contain identical
DNA. But it takes much more splitting to make an embryo. As the
cells split again and again, they can make mistakes in the number of copies of a
given gene they pass to daughter cells. One twin's splitting may err and
the other not (or err in a different way) for a given gene. That's how
twins can arrive at different numbers of copies of a gene, and why the DNA of
some identical twins are not identical.
Actually, this is a simplified account. Some identical twins do form on
Day 1 from the initial split of the fertilized egg; they are the most alike
twins. Most identical twins, however, form from a common cell mass on
about Day 4 (and are mirror images of each other). But the idea is the
same: repeated splitting of cells can introduce errors in the number of
copies of a given gene passed to the daughter cells.
By the way, although most DNA differences due to CNV do not effect health or
development, some may be a factor in developing a disease, says Bruder.
One of the twins Bruder studied lacked certain genes linked to leukemia risk.
Indeed, that twin had leukemia. His twin, who did not lack those genes,
did not have leukemia.
(Answered April 10, 2002; updated April 10, 2008)
Further Reading:
Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA
Copy-Number-Variation Profiles, by Carl E.G. Bruder et al. The
American Journal of Human Genetics,
Volume
82, Issue 3, 763-771, 14 February 2008
Twins
trigger: In the case of identical twins, what triggers the fertilized egg to
result in two embryos? What's the probability of having identical twins?
Forensic-Evidence.com:
Different fingerprints
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