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.