Even though identical twins are from the same sperm and egg and therefore have exactly the same set of chromosomes and therefore genes, researchers are realising that there are other forces at play – the science of epigenetics.
Even if 2 people have the same set of genes, because of the different environments they experience – i.e. in the womb (maybe one got more nutrients than the other), the types of illnesses they have had as they grew up; the lifestyle they lead, etc, there can be differences in how much each twins' genes are turned on or off – this can then affect the way each twin looks and behaves. This effect increases as the identical twins get older, because they are exposed to more and more diverse environmental events (different illnesses, jobs, friends, habits etc).
Silencing of one X-chromosome is also responsible for differences in female identicals. When a female foetus is created; it has 2 copies of the X chromosome (one provided by Mum’s egg; one by Dad’s sperm; a boy has one X from Mum and a Y from Dad). The version of each gene that is ‘dominant’ is the one that is expressed, but one of the two X chromosomes is permanently ‘silenced’ very early in embryonic development so the only genes that work are the ones on the X chromosome that is not ‘silenced’. This silencing can happen AFTER the identical twins have been created and are developing (when the embryo has grown to about 10-20 cells). If by chance one identical twin ‘silences’ the X chromosome that came from Dad’s sperm and the other twin silences the X chromosome that came from Mum’s egg, then they have different genes working in their systems, which can result in noticeable differences.
Did you know that having one placenta doesn't necessarily mean that twins are identical?
Parents of twins have routinely been told that if their twins were born with the one placenta that they were identical however in recent years there have been a few reports of twins who were told they were identical for precisely this reason - only to later find out that they are in fact non-identical. This is due to something called 'chimerism' - when an individual is composed of 2 genetically different types of cells. It is thought that approximately 8% of non-identical twins are chimaeras; the rise in IVF is considered to be a contributing factor. A report has just been published which highlights a case study of chimaeric twins.
If you would like to know more about having a test done to see if you or your twins are identical or not, please see How do you know if you are identical or not?