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.
So when one egg is fertilized by single sperm then identical twins or mono zygotic are born and they are always of same sex. But when two different eggs are fertilized by two different sperms then fraternal twins are born. In fraternal twins sex of both twin are irrespective of each other .