What is the difference
between Identical and Fraternal:
Identical (monozygotic) twins result from one fertilized egg that splits into
two, resulting in same sex twins who share 100% of their DNA. There is no
genetic marker and no explanation scientifically as to why this happens.
No one knows what causes identical twinning. The babies will have
identical DNA with the only varying identifiable factor being slightly
different fingerprints.
Fraternal (dizygotic) twins are thought to be genetic and this
predisposition can cause women to be more likely to release more than one egg
at a time, offering the chance for more than one egg to be fertilized at a
time. Other factors that can increase your chances of having fraternal
twins include using fertility treatments and a mother being over 35-40 years
old when she becomes pregnant. These twins share only 50% of their DNA,
just like regular siblings, and can be same gender or boy/girl.
How common are twins?
The rate for identical
twins/multiples is random and universal; it's the same for all countries and
ethnicities and this has remained constant over time. The chances of
having identical twins is about 1 in 285. Of the identical twins, half
are male/male, and half are female/female. If one of your parents is an
identical twin or if you (or your partner) is an identical twin, your odds are
no higher for having twins than anyone else.
Do twins run in
families?
There is no evidence
that identical twins run in families. However, fraternal twins do run in
families. A woman can inherit the twin gene directly from her mother or from
her paternal grandmother through her father. These women may carry a gene for
hyperovulation, which means they release more than one egg during an ovulation
cycle, increasing their ability to conceive fraternal twins. This is why many
people believe that twins sometimes "skip a generation."
Having twins in the family does not make a man more likely to father twins
himself, as his genes have absolutely no influence on whether his wife's
ovaries will release multiple eggs. However, a man with fraternal twins in his
family can pass the twin gene on to his daughter, increasing her likelihood of
releasing multiple eggs and conceiving fraternal twins.
Mirror Image Twins
Mirror-image twins
occur only in identical twins. In approximately 23 percent of identical twins,
the fertilized egg splits later than 7 days following conception. In this
case, the original right half of the egg becomes one twin 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.
Famous Celebrity Twins
Elvis Presley (twin
died at birth), Deidre Hall, Jill Hennessey, Isabella Rosellini, Ann
Landers, Mario Andretti, Kiefer Sutherland, Alanis Morrisette, Vin
Diesel, & John Elway.
Famous Parents of
Twins
Pavarotti, Chuck
Norris, Al Pacino, Margaret Thatcher, Cleopatra & Mark Antony, George Bush,
Steve Forbes, Mel Gibson, Muhammad Ali, Ron Howard, Robert DeNiro, William
Shakespeare, & Denzel Washington.