Mirror twins are siblings who develop simultaneously in their mother’s womb from the same egg and sperm cell and appear physically identical. They are more commonly known as identical twins; the scientific term is monozygotic twins. Other twins develop through different means and are not identical. These are known as fraternal twins. Identical triplets and quadruplets, while rare, are also possible.
Identical twins form early in fetal development, when the fertilized egg is not yet an embryo, but a collection of cells called a zygote. In the process of becoming an embryo, the zygote can sometimes be broken into two groups of cells. Since these are stem cells, which can form into any other kind of cells, the two groups simply develop into two identical embryos. Barring any complications, they are born as two separate persons with nearly identical physical features. A similar process accounts for identical triplets and quadruplets.
A different process creates fraternal twins. They develop from two separate fertilized eggs, not from a single egg. Consequently, fraternal twins can have great physical dissimilarities, including different genders and, occasionally, even different racial characteristics. Mirror twins, on the other hand, are always of the same gender.
The term mirror twin is usually quite accurate. These twins will share irregularities such as moles or birthmarks, but often on opposite sides of the body from the twin’s features. Aside from these differences, identical twins are almost indistinguishable, although immediate family members usually learn ways to tell them apart.
Mirror twins are not completely identical. For example, they do not have identical fingerprints. In 2008, scientists discovered that these types of twins do not share identical DNA.
In addition, one twin may be prone to certain genetic diseases, but not the other. This factor could be an asset to determining the genes responsible for such diseases. Even before this discovery, identical twins were sought-after subjects for scientific study because they have the same genetic backgrounds but different experiences and environments. Mirror twins have provided numerous insights to sciences ranging from immunology to sexuality.
Famous mirror twins include advice columnists Ann Landers and Abigail Van Buren and Canadian musicians Tegan and Sara Quin. Athletes Jose and Ozzie Canseco and Ronde and Tiki Barber are also identical twins. Mirror twins have been popular fixtures of myth and storytelling since antiquity. Romulus and Remus, the legendary founders of Rome, were identical. In fiction, they often symbolize humanity’s dual nature or other aspects of appearance versus behavior, giving rise to the evil twin mythos. They are popular on soap operas as dramatic devices, sometimes allowing popular actors to return after their characters have been killed off.