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.
Fraternal twins and identical twins differ in how the set of twins arise. Fraternal twins occur when a woman releases two eggs during ovulation instead of just one (which is what is normally released). Each of these eggs can then be fertilized by a different sperm cell from the father.