Heredity of twin births and different twin types

“Do twins run in your family?” This has probably been one of the most asked questions – by friends as well as strangers – since the very moment we learned we were expecting twins. What follows is usually me giving a brief explanation of why it doesn’t really matter if there are twins in our families or not, as identical twins (like our girls) are never hereditary. People then usually stop asking, but most of the times I can see by the look on their face that either they didn’t quite understand or they just don’t (want to) believe it. So I’ll take a moment here to explain a bit more about the heredity of twins and the differences between identical and fraternal twins.