Identical twins are always boy/boy or girl/girl But if that egg split happens on days 2 or 3, before the sac has started to form, each zygote develops its own sac and A boy and girl twins cannot be identical Monozygotic or identical twins are usually of the same sex as they are formed from a single zygote (fertilised egg) which Another site said that 1/3 of all twins are identical Others give different numbers If I take that first pair of numbers, we get the probability of two boys, or of two
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