Â· 4 years ago. A mutation is a change in the genetic material of an organism. We are made from trillions of cells, each with a nucleus composed of DNA â a set of instructions that tells the cell what to do. Cells copy themselves with astonishing accuracy, but every now and then a piece of code is copied incorrectly.
Identical (or monozygotic) twins happen when a fertilized egg splits into two separate babies-usually within the first twelve days of Mom's pregnancy. After twelve days, the egg may not split entirely and result in conjoined twins.
If the split occurs between the 4th and 8th days, then the twins will share the same chorion but have separate amnions. Monochorionic diamniotic is the most common form at monozygotic twins, accounting for 75% of monozygotic twins. If a split occurs after the 8th day and before the 13th day, then twins will share the same chorion and amnion. This is a very rare condition and accounts for 1-2% of monozygotic twins. The embryonic disk starts to differentiate on the 13th day.