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.
You actually CAN'T tell for sure if they are fraternal or identical until they are born if they don't share a sac and have 2 placentas. If the egg splits early on (like in the fallopian tube) and then implants the babies can be identical (one egg that split) but have 2 placentas and be in 2 sacs.