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 develop when two eggs are fertilized. So if older women are more likely to produce two eggs per cycle, they're also more likely to have fraternal twins, the researchers argue. The study doesn't cover identical twins, which are much rarer and develop from one fertilized egg.