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Facts About Twins
May 2009
If you've had twins, you can usually tell if they're identical or fraternal. Identical twins, or monozygotic twins, come from the same egg that will later split in two. If your twins look like copies of each other, you probably have identical twins. These twins will share 100% of their DNA, have the same hand and footprints and yet have different teeth marks and fingerprints.
Fraternal Twins
Fraternal twins are dizygotic, that is, they come from two different eggs, each fertilized by different sperm. That mean they share only 50% of their DNA, much like all other singleton siblings. They can be the same or opposite sexes.
Interestingly, they can as dissimilar as some singleton siblings OR they can look like identical twins! Did you know Mary Kate and Ashley Olson are actually only fraternal twins, yet look identical? The only way to be sure whether your twins are truly identical, then, is to take a genetic test to see the overlap of DNA.
Mirror Image Twins
About 25% of identical twins are mirror image twins. Their hair falls in opposite directions, they have mirror image fingerprints and if one is right handed, the other is left handed.
Other Types of Twins
While identical and fraternal twins usually hog the spot light, there are a few other types of twins that are equally intriguing. Conjoined ("Siamese") twins are those that are attached at birth. Half identical twins, or polar body, occur when one egg splits in two before fertilization. Each half is then fertilized by a separate sperm. These twins share 75% of their DNA as they share the same DNA from one egg and different DNA from each sperm.
Who Will Have Twins?
Are there signs that determine whether a woman is more likely to have twins? Yes. A woman who is of above average height and who has a large bone structure is more likely to have twins. So are African Americans, women who give birth at an older age (30+) and women who've taken fertility drugs or been on birth control for several years. Lastly, if a woman was a fraternal twin herself, the chances of her having twins increase fivefold.
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