In your life you’ve probably seen twins. Two individuals who look extremely similar to each other.

Once the embryo is formed, it begins to divided inside. So far the women will only have one child.

However once in every 250 cases, the egg does something amazing, it splits in two.

This creates two separate eggs, with the exact same genetic information.

They are refereed to as Mono zygotic twins (mono - one) because they are formed from one egg. This whole process usually occurs within roughly the first 12 days of conception.

In some really rare cases, you can get mono zygotic twins, who are male and female.

In rare cases the egg will may contain 3 sex chromosomes (X X Y). When the egg splits to form 2 mono zygotic twins, sometimes a sex chromosome is lost, resulting in one male and one female twin.

Fraternal twins occur when the women releases  two eggs, each fertilised by two different sperms. Although they are born at the same time, they are genetically different.

As similar as brother and sister.

What you might also see are conjoined twins.

When it comes to identical twins, the egg splits into two complete identical cells, but in conjoined twins, the cell dose  not completely split.

This means the parts of the cell which have split, grow independently of each other, while the parts still together grow as one.

The case to the left is of conjoined twins Abby and Brittany Hensel

Their case is extremely unique, as most other people in their situation pass away at birth.

Although it make look like their joint at the neck, if you look inside, they have two different hearts, lungs and stomachs. From the waist down however, they are conjoined.