Why are non-identical twins genetically different but identical twins genetically the same?

Answer 1

Identical twins come from the same egg and sperm so they have all the same DNA. When the egg is fertilized it starts splitting to form new cells which is how the embryo grows. Sometimes when the egg splits it comes totally apart creating two embryos with one cell instead of one embryo with two cells. Both of these grown into fully formed babies but since they share all the same DNA they look identical.

Fraternal (non-identical) twins are from two different eggs and two different sperm. When a woman ovulates (releases an egg) she usually releases just one. Sometimes she will release two (or more) at the same time and if they both get fertilized then two babies will be born. These two babies are not any more closely related genetically than any other brothers and sisters are.

Answer 2

Although identical twins are formed from a single original cell, there can be subtle differences in their gene encoding, including a variation in the presence or number of gene copies found in various regions of the genetic code (copy number variants). There is also a probability that outside influences will make the genes of some cells different long before birth, as well as in infancy and childhood.

That said, the vast majority of genes in identical twins will be indistinguishable between the two.

Answer 3

Non-identical twins (fraternal twins) result from two separate eggs fertilized by different sperm and share, on average, 50% of their genetic material. Identical twins (monozygotic twins) come from a single fertilized egg that splits into two embryos, making them genetically identical as they share 100% of their genetic material.