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.
In addition to all the different combinations of identical and fraternal multiples, there are some rare types of twins. Mirror-image Twins. Mirror-image twins occur only in identical twins. In approximately 23 percent of identical twins the egg splits later than usual, most often day seven or beyond.