Twins are formed by either one fertilized egg splitting into two, or two eggs being released by the mother and then fertilized.
Identical twins occur when a single egg is released by the mother and fertilized by one sperm. At one point, after the egg has been fertilized, it splits into two halves that are genetically identical. These twins share the same sex, eye color, hair color, and so on. This is called a monozygotic birth. It is unknown what causes the egg to split, or at exactly when the splitting occurs.
Fraternal twins are created when the mother releases two eggs. Each egg is then fertilized by a different sperm. This is called a dizygotic birth. Fraternal twins may be of the same sex or different sexes. Only 50% of their genes are shared, making them much like any other two siblings.
Conjoined twins occur in a similar manner to identical twins, except the egg takes considerably longer to begin splitting. As a result, the division stops before the two halves have completely separated, and the twins develop physically attached to one another. Sixty percent of conjoined twins are stillborn or die in the uterus.
A fourth type of twin, called half-identical twinning, is still being researched by scientists. It has not been proven to occur in a human. A small part of a single egg breaks off before fertilization occurs. If it survives and grows, it may act as a second egg.
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