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 nature, twins occur just after fertilization of an egg cell by a sperm cell. In. rare cases, when the resulting fertilized egg, called a zygote, tries to divide into. a two-celled embryo, the two cells separate. Each cell continues dividing on its. own, ultimately developing into a separate individual within the mother.