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Twins
When we think of twins, we usually think of identical twins. Identical twins are born together, they are of the same sex, and they look exactly the same. In fact, they look so much the same that people often have difficulty telling them apart.
However, not all twins are identical twins. Fraternal twins, like identical twins, are born together, but they are not always of the same sex and they do not look exactly the same. Sometimes the only way to tell fraternal twins from other brothers and sisters is to find out their birth date.
Identical twins look exactly the same because they are formed from the same egg, or zygote. Thus, identical twins are also known as monozygotic twins (mono=one, zygote=egg). Because they are formed from the same egg, monozygotic twins have the same genetic characteristics. Fraternal twins, on the other hand, are formed from separate eggs, so they are called dizygotic twins. (di=two). They come from different eggs, so their genetic characteristics are different.
Because they did not understand how twins are formed, people in the past were afraid of twins. In some societies, twin babies were killed or left for wild animals to eat. According to one legend, the city of Rome was founded by twin brothers who were left to die but brought up by a wolf. Today, the fear of twins is gone, but people are still interested in them. Scientists are particularly interested in studying twins. They hope to find out more about genetics and heredity from twin studies.
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