Overview: What Are Twins?
Twin births, in which a single pregnancy results in two fetuses, are the most common form of multiple births. Most of the time, human babies are born one at a time,. but about 33 times out of one thousand births per year in the United States, twins are born. Fraternal twins are more common than identical twins.
What Are Identical Twins?
Identical twins are also called monozygotic, because they develop from a single fertilized egg. When the fertilized egg first begins dividing, it forms 2 separate embryos within the same fetal sac. Most often, they share the same placenta also. Identical twins are genetically identical, except for random mutations that can affect one twin rather than another. In addition, they are nearly always the same sex, with female pairs more common than male pairs. Some pairs of identical twins have mirror-image traits, in which one twin will be left-handed while the other twin will be right-handed, or one will have a mole on one side of their face, while the other twin will have an identical mole on the opposite side.
What Are Fraternal Twins?
Fraternal twins are also called dizygotic, because they develop from two separate fertilized eggs which are released at the same time. When the eggs begin dividing, they form two separate embryos, usually within 2 fetal sacs. They are no more genetically alike than any other siblings with the same parents. Fraternal twins can be a male-female pair (the most common), a female pair, or a male pair. They may look alike, or they may look different from each other.
What Are Twin Studies?
Scientists study pairs of identical twins in order to gain insight on how traits may be expressed. Identical twins who are reared apart are the focus of many of these studies, because the twins have largely identical heredity, but their environments are different. There are many examples of twins who meet after bring separated who have similar names and have similar occupations, traits that would not seem to be determined by genes. Other expressions, such as similar disease processes, as well as height, eye color, hair color, and skin tone, are more clearly genetic.
Any Other Features of Twin Development?
The language development of twins in the same family can be delayed, because most twins develop a language of their own that they can speak and the other twin will understand. They can communicate well with each other, so they don’t need to develop language to communicate well with other. Twins in the same family will often try to develop their own individual interests, preferences, and personality, in a process known as “twinning”, so that they are sometimes even less alike than other siblings in the same family.
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