Question

why some twins are not alike ​

Answer 1

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Because they are not identical.

They are dizygotic twins.

Answer 2

the twins are of two types -

1) Monozygotic twins

2) Dizygotic twins

in Monozygotic twins the gender and its all similarities are same

in Dizygotic twins gender is different

so, if they are Dizygotic twins then they will not alike