|What makes some twins of the same sex identical
and others not?
|Question Date: 1997-11-07|
I am amazed at this question because I have twins
daughters in the 3rd grade at Roosevelt School--so
it is funny that you asked me this
I am a geologist and an
astronomer (an astrogeologist) and so I am not an
expert on biology. But this is how I understand it
Twins that are called IDENTICAL form
when the egg in the woman spontaneously splits
into two when it first starts growing inside the
woman's uterus. So in this case, a single egg
splits into two and the twins that form are
identical twins. Identical twins are always the
same sex...2 boys or two girls, NEVER one of each,
because there was originally only a single egg
within the mom before it split into
The other kind of twins are called
FRATERNAL TWINS. In this case the woman makes TWO
INDEPENDENT eggs (called multiple ovulation)
inside her body and each of these eggs grows and
develops into a baby. The eggs were NEVER THE
SAME. Hence fraternal twins can be the same sex or
different sex; there is a 50 % chance that they
are the same.
The difference between identical twins and other
twins is in the first stages of
fertilization.Usually, a woman produces a single
egg, which, if fertilized by a man's sperm, can
produce a single child. The woman's egg has 1/2
of the genetic material of the mother, and is
joined with the sperm, which contains 1/2 of the
genetic material of the father. But no two eggs
or sperm are just the same. In making the sperm
and the egg, the genes are chosen more or less at
random from the possible choices of both mother's
and father's genes. It's like flipping a coin to
make a choice between two things - it is tough to
predict what you will wind up with. By mixing
together some of the genes of the mother and some
of the father, we get a whole new genetically
distinct person - the child. This is why everyone
is different - or almost everyone except identical
Twins can occur in two distinct
ways.If a woman produces two eggs during a single
cycle, both eggs can be fertilized by two
different sperm. Each egg has a distinct genetic
code, as does each sperm. So, while the two twins
share the womb, they do not share the same genetic
material, so they can be distinctly different.
One can be a boy and the other a girl, or both can
be boys or girls. It is just that they shared the
mother's womb for a time that makes them twins.
For identical twins, it is quite
different. Identical twins start out as one egg
fertilized by a single sperm. All things grow by
a process of cell division - one cell divides into
two, two into four, etc. Early in this process of
cell division, the cells that will make identical
twins split into two distinct groups.
Basically, what should have been one individual
becomes two separate individuals. But because
they came from the same egg and sperm, they have
exactly the same genes. So, the two will develop
identically and become identical twins. Identical
twins are more like each other than any other
human being is like any other human being.
The mixing of genes that occurs during
fertilization is also one reason why everyone is
so different. Each of us has a little bit
different genetic material than everyone else.
Genes are the instruction set for how we grow, how
big, how much we weigh, what color skin and hair,
and so on. This mixing of genes occurs every
generation, and is what makes living things so
The genetic code for a female is XX, the genetic
code for a male is XY. The sex of a baby is
determined by the father's sperm. All eggs inside
a mother are "X". The father's sperm is either X
or Y. When the egg and sperm combine they create
either a boy XY, or a girl XX. So within the
father, there are millions and millions of sperm,
some X, some Y. So any father can produce as many
boys or girls as need be. It all depends on which
type of sperm gets to the egg first, an X sperm or
a Y sperm, that determines whether the baby will
be a boy or a girl.
When twin babies develop,
it can be via one of two ways. Either, the mom
can produce 2 eggs at once which each get
fertilized by a sperm. These are called fraternal
twins. Since they are two totally separate eggs
and separate sperm, they can be either one boy,
one girl, or both girls, or both boys. Since
these twins arise from separate eggs and separate
sperm, each baby is genetically different from
each other. It's just like having a double
The other way twins can develop is
by a single egg getting fertilized and then as
that embryo begins to develop, it spontaneously
splits into two totally separate embryos and they
both develop simultaneously each as a whole baby.
Since in this case it is all begun with one egg
and one sperm, these types of twins will be always
the same sex, either both boys or both girls,
depending on whether an X or a Y sperm fertilized
the egg. These twins would be known as identical
twins, because they have the exact same genetic
What about triplets or quadruplet
babies? Could you have identical and fraternal
twins in each of these cases?
There are 2 ways to get twins, which is why there
are 2 kinds of twins:
Fraternal twins happen when 2
different embryos start growing at the same time,
so you get a brother and a sister or 2 sisters or
2 brothers that are the same age.
Identical twins happen when 1 embryo
splits into 2 embryos when it is only a few cells
big, so both babies have the same genes.
guess triplets and quadruplets and more-lets
probably aren't usually identical, because it's
hard to imagine an embryo separating into more
than 2 pieces that can each develop into a baby.
I wonder what is the largest number of identical
babies that's ever been born from a woman. Do you
think there are ever identical triplets? identical
quadruplets? Maybe identical quadruplets are more
common than identical triplets, because the way
the embryo grows is that 2 cells divide into 4
cells, which divide into 8 cells, so it seems like
embryos would be more likely to split into halves
and quarters than into thirds.
I wonder how
many cells big the embryo is when it splits into
twins. I'd guess most embryos split into twins
when they are 2 cells big, and some when they are
4 cells big, and a few when they are 8 cells big,
but that is only a guess.
When the embryo
develops, it starts out as a ball of cells. Then
the cells move so that they're all on the surface
and the embryo is like a hollow ball. Then the
ball collapses and sort of folds in half. That's
when the exciting changes take place, and cells on
different parts of the embryo turn into different
things like nerves and skin and gut.
luck with your science, and let me know if you
have any other questions.
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