This is one of the most pondered concepts of twinning and it lends to the unique insight of the answer to the 'nurture vs. nature' question. This is why there are many studies done on identical twins - they provide a control group for genetic factors into disease and personality difference so that it is possible to have insight into how much of a disease is caused by genetics versus how much of it is attributable to an environmental element.
|Ingrid & Helen (Identical Twins) age 2|
Twins who came from the same fertilized egg are considered ‘monozygotic’, or ‘identical’ twins. They are begotten of 1 egg and 1 sperm, which after joining and growing for a time, divided in half, creating two blastocysts. There are a few exceptions to the above traditional origins of monozygotic and dizygotic twins, but they are mostly related to genetic mutations that are not common, and I won’t go into explaining them. If you want more info though, check here (see 'Half-Twins' Heading).
|Helen & Ingrid age4|
Dizygotic twins are no more genetically alike than any brother and sister. They originate from 2 separate eggs, which are fertilized by 2 separate sperm. That is why you should never ask a twin mom who has boy/girl twins if her twins are identical! (yes, some people do ask this!) some interesting exceptions to this include the possibility that fraternal twins could actually be the children of two different fathers! (Though I don't recommend this!)
One interesting note is that even though monozygotic twins are referred to as 'identical' twins, thier DNA is not completely identical. This is because there is a small amount of genetic material that is stored in the plasma of cells (not the nucleus where the most DNA is). So when the newly fertilized egg divides, one twin will end up with some mitochondrial DNA from the cell plasma and the other side will have the rest of it.
|Ingrid & Helen age 5|