The answer to this question depends on what type
of twins they are! There are two types of twins.
In one case, twins can be produced when two eggs
are present at the same time inside the mother,
and both of these eggs get fertilized. Each of
them eventually grows into a separate baby.
Because each egg and each sperm started with
different DNA, the babies that grow from each have
different DNA. These twins are called fraternal
twins, and they are just like other siblings,
except that they were born at the same time!
(This is why sometimes you might meet a brother
and sister who say they are twins). In the other
situation, there is only one egg, and it is
fertilized by only one sperm. Normally the
fertilized egg would divide and grow to form one
baby. However, sometimes after the egg is
fertilized and starts to divide, it can split
apart. After it splits apart, each half can
continue to divide and grow into a baby! So, in
this case, the two babies came from one egg.
These kinds of twins are called identical twins--
and yes, they do have identical DNA, because they
came from the same fertilized egg! You can
remember it this way: fraternal twins each have
their own DNA, while identical twins share
Even though identical twins have
the exact same DNA, though, they aren't really
perfectly identical. That's why every mother can
tell her identical twin babies apart. The reason
they aren't identical is that each of them
develops separately, and lives a separate life.
For example, if one of them has an accident riding
his bike, then he might injure himself and get a
scar, while his twin brother does not have one.
Also, some things in development are a little bit
random, and not just due to DNA. For example,
identical twins have similar, but not identical,
Identical do have the same genotype. After all,
they come from the same fertilized egg. Recent
studies have shown that identical twins have very
"similar" not "identical" DNA, but for the most
part, according to basic biology it is identical.
Identical twins aren't completely identical
because DNA is essentially like instructions to
building something, how your body decides to build
that is random. This is why identical twins can
have differing fingerprints. Another big factor
why identical twins aren't necessarily completely
identical is the environment which each of them
were raised in. But for the most part, basic
biology says identical twins share the same DNA.
More research is being done to investigate this.
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