I find the genetics of black/white twins fascinating.
http://www.mirror.co.uk/news/real-life-stories/black-white-twins-meet-sisters-5256945
They can be almost identical but different skin colour due to black/white/mixed race parentage.
There are varying types of twins. One type is a single cell splitting which results in identical twins, or mirror twins. Then there are twins where there are a couple of eggs fertilised such as boy/girl twins, although a friend of mine has twin girls who aren't a bit alike, but were similar...