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If two eggs are released from the ovary, or if one egg is released from each ovary, and are fertilized by two separate sperm, fraternal twins (also known as dizygotic twins) have been conceived. Fraternal twins can be the same sex or different sexes, and will look as much alike or as different as any siblings.
If one egg is fertilized and later divides in two eggs, identical twins will develop. The same goes for an egg that is fertilized and later divides into three eggs, which will result in identical triplets. Identical multiples have the same chromosomal makeup, so they will be the same gender and have the same hair color, eye color, blood type, etc.
Only one-third of all twins are identical (also called monozygotic twins). In the case of multiples, any combination is possible. For example, if two eggs are fertilized at the same time, one fertilized egg can split into identical twins, resulting in the birth of triplets -- one single and two of which are fraternal twins.