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.
I've checked out some books and some Web sites on twins. I believe what I'm reading is that there is a range of time during which a fertilized egg can split into identical twins, and this spans the time during which the egg implants. Thus, twinning can take place either before or after implantation.