Question: What is the difference between identical and non-identical twins?

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  1. Great question, katnissmar!

    The difference is that identical twins come from one sperm and one egg, and that non-identical twins come from two eggs and two sperms.

    Non-identical twins happens when the ovary (which holds the eggs) accidently lets two eggs out instead of one. We call it “ovulation” when the egg is let out of the ovary. and this happens before the sperm and the egg meet.

    in identical twins, one egg and one sperm meet and make an embryo, and then the embryo breaks into two. we have absolutely no idea why this happens, but we do know that it happens more when we grow the embryos in a dish (we call this IVF or “in vitro fertilisation”). we don’t know why that happens either.

    The other difference between identical twins and non-identical twins is that identical twins, because they had the exact same sperm and egg to begin with, have exactly the same DNA (or genetic code), and non-identical twins do not. this is why, identical twins always have to be both girls or both boys, but non-identical twins can be boys, girls, or one of each! 🙂

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