Could You Have an Identical Twin?
I am not a twin myself, but if I were I would definitely want to
be an identical twin. There is just something amazing about
having someone else in the world with my exact DNA, down to the
last amino acid. Most people think of twins as being either
identical or fraternal, but there are many different types of
twins.
Identical Twin - Identical twins are born when a
fertilized egg, or zygote, splits in two. Because the egg is
fertilized by one sperm cell, and it is the sperm cell that
determines the sex of the child, identical twins are always the
same sex.
The two children share 100% of their DNA, meaning that the most
complex DNA test could not tell the difference between the two.
There are differences, however, in the phenotype, or physical
expression of the DNA, that helps people distinguish between one
identical twin and the other. Though they share the same
genotype, they have different teeth and finger prints, for
example.
Half- Identical Twin - Half- identical twins are born
when the egg splits prior to being fertilized, and the two new
cells are fertilized by two different sperm. The siblings share
25% of their DNA.
Monoamniotic Monochrionic Twins - Depending on how long
after conception the egg splits in two, identical twins may
share the same yolk sac, chrionic sac, and amniotic sac. Twins
that share all three sacs are known as monoamniotic monochrionic
twins, and there are several health risks that are associated
with this situation. There is an increased risk of tangling with
an umbilical cord, because the identical twin not only has to
worry about her own cord, but also that of her sister. The twins
have a greater risk of TTTS (Twin to Twin Transfusion Syndrome)
where the placenta is unevenly shared - thus, one gets too much
blood and the other gets too little.
Mirror Twin - Some 25% of identical twins are mirror
twins. They have mirror image finger prints, one is left-handed
and the other is right-handed, and their hair spirals in
opposite directions.
Conjoined Twin - Commonly called Siamese twins, they are
monoamniotic monochrionic twins who never completely separate.
Thus, when they are born they share some part of their body.
Often their hips or shoulders, and more rarely their heads, are
connected. The majority of conjoined twins are mirror twins.
Several successful surgical separations have been performed,
allowing the twins to live normal lives.
Fraternal Twin - Far more common are fraternal twins, who
come from two different eggs fertilized by two different sperm
cells. They share 50% of their DNA just as other siblings born
at different times do.
Twins of Two - When two separate eggs are fertilized by
two different men, twins of two are conceived. The children are
genetically half-siblings and share just 25% of their DNA.
One hundred years ago, the average parents of twins would have
had to guess whether they were identical or fraternal. Nowadays,
in addition to sophisticated medical equipment available,
parents can have DNA tests done to determine what type of twins
their children are.