Are identical twins actually 100% DNA identical?
In general the answer is yes. At least their DNA is as similar as any two people’s can be (other than clones which have not yet been raised). MZ twins were conceived from a single zygote (fertilized egg) of which half of its DNA came from the father and half from the mother. When the eg split, both halves of the division had identical DNA and so the twins develop with this identically matching DNA. One clear exception is the mitochondrial DNA which is always different in MZ twins and can also cause some biological differences. For many years MZ twins have been used to determine whether certain conditions, diseases, and traits were genetic (nature) or environmental (nurture). The assumption being obvious that if both twins had the condition, it was most likely due to their common genes. If only one had the condition it was due to their environment or upbringing, or phenotypic differences (those caused by different embryology and development such as fingerprints and many other traits). B
