What is Genetics? : Frequent Questions & Answers

Are identical twins 100% genetically identical?

It is true that identical twins share their DNA code with each other. This is because identical twins were formed from the exact same sperm and egg from their father and mother. (In contrast, fraternal twins are formed from two different sperm and two different eggs.) Usually, the egg and sperm come together and create an embryo, which eventually grows into a human baby. However, in the case of identical twins, early in development, this embryo divides into two. This creates two babies rather than one.

While the two babies share the same DNA code, there is more to our genetics than just that. During development in the womb and after birth, our surroundings, exposures, and nutrition influence how our genes are expressed and how our bodies and minds develop. For example, two identical twins may have the same genes for height, but if one twin does not receive the same amount of nutrients while in the womb, it may be shorter than the other twin. We know also, that there are some changes to our genes that can happen during the embryonic period or during development. While this rarely happens, it makes it so that one identical twin may have a genetic condition, while the other twin does not. Our genetic code is a very large part of our genetic makeup, but there are other factors, genetic and environmental, that make us unique!