Ever
heard of twins who celebrate their birthdays months apart? What about
twins who have different biological fathers? This seems impossible, but it
turns out that both of these things happen. These incidents are rare, but
real.
As we mentioned earlier, some women can release more than
one egg during ovulation. In some exceptional instances, these eggs are
released at different times. Perhaps at 12 or even 20 days apart. Now,
let's say both of these eggs are fertilized shortly after they are
released. The woman conceives twins, but on two separate dates. Since the
dates of conception differ, so will the dates of delivery. The twins could
be born a couple of weeks or a month apart. This is known as
superfetation.
When a mother releases more than one egg during
ovulation and has intercourse with more than one male during that time, it
may result in superfecundation twinning. Two distinct sperm from two
separate individuals fertilize the two eggs the woman ovulates. Obviously,
the twins resulting from superfecundation are dizygotic.
Some
research suggests another type of twin called "polar body" or
"half-identical" twins. This type of twin is supposed to occur when one
egg splits before fertilization and is then fertilized by two separate
sperm. It is believed that these twins share 75 percent of their
Original text: http://science.howstuffworks.com/twin5.htm
MIRROR-IMAGE
TWINS
Mirror-image
twins occur only in identical twins. Here is how it happens: in
approximately 23 percent of identical twins the egg splits later than
usual, most often day seven or beyond. The original right half of the egg
becomes one individual and the original left half becomes the other. These
twins will often have "mirror images" of their features, such as hair
whorls that run clockwise in one and counter clockwise in the other, a
birthmark on the right shoulder of one and the left shoulder of the other,
etc. There is no specific test for determining if twins are mirror-image.
The determination is made by observation only, and the twins must be
monozygotic, or identical.
This may be a partial explanation for
the fact that a little over one third of identical twins are left-handed,
double the rate in the general population. In extreme cases all of the
internal organs are reversed in one of the twins, with the heart on the
right, the liver on the left and the appendix on the left.
POLAR BODY TWINS (HALF-IDENTICAL)
Polar body twinning is
very unusual and very rare. The process is quite complicated. The polar
body appears when the egg has been developing, even before fertilization.
It is a small cell that does not function and will usually degenerate and
die. It is thought that in some cases, when the egg is old, the splitting
off of the polar body takes place in an abnormal way. It then becomes
larger, receives more nourishment, and does not die as it usually does.
Instead, it acts as a second egg. The polar body and the egg share
identical genes from the mother, but they may then be fertilized by two
separate sperm from the father. This will result in twins who share half
their genes in common (from the mother) and the other half different (from
the two sperm). They share some features of identical twins and some
features of fraternal twins and thus are so-called half-identical
twins.
MIXED CHROMOSOMES OR CHIMERISM
Another form of
twinning that has been identified is called chimerism. This is thought to
occur if two separate sperm fertilize two separate eggs which then fuse,
producing individuals with different sets of chromosomes. Some have been
identified that have more than one distinct red blood cell type and
individuals who are both XX and XY (the sex chromosomes - XX being female
and XY being male.) This phenomenon might also be associated with fused
placentas causing intermixing of the circulations. It is very rare, and
fewer than twenty-five cases have been identified. It is more common in
other mammals, such as calves.
Original text: http://www.nomotc.org/library/twinning_facts.html