There are three types of identical twins: Monochorionic twins are more typical when it comes to identical twins, while ...

Because identical twins occur when a single fertilized egg splits and forms two embryos, each embryo has the same chromosomes—including sex chromosomes (such as XX for females and XY for males).

That's why fraternal twins don't look identical, and can be boy/girl rather than just girl/girl or boy/boy. The majority of twins (70%) are fraternal. Learn your chances of having twins.

As it turned out, the twins were neither fraternal nor identical. They fell into a third rare category known as semi-identical or sesquizygotic twins. Although it would take several years to prove ...