http://www.wisegeek.com/what-is-the-difference-between-monozygotic-and-dizygotic-twins.htm
http://answers.yahoo.com/question/index?qid=20061119133111AAEtCnl
Fraternal occur when the mother has two eggs that are fertilized at the same time. They can be boy/boy, girl/girl or boy/girl.
Identical twins occur when the egg splits into two embryos after it has been fertilized. They have to be of the same sex.
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