SCIENTISTS have found evidence of effects before birth that could account for some of the differences between identical twins.
On paper, those that share the same womb and set of genes should be indistinguishable. But the activity of their genes can be affected by unique environmental factors.
Researchers identified widespread differences of this kind in the genes of identical twins at birth.
They believe they can be traced to the influence of tissues such as the placenta and umbilical cord that are specific to each twin. Genes are affected by environmental factors through a chemical process called epigenetic tagging, which acts like a switch turning them on or off.
In the new study, scientists looked at the epigenetic profiles of umbilical cord and placenta tissue, and cord blood from pairs of new born identical and non-identical twins.
Even in identical twins, they were significantly different.
"This must be due to events that happened to one twin and not the other," said lead research Dr Jeffrey Craig, of the Murdoch Children's Research Institute in Australia. Further evidence supporting the known link between low birth weight and the risk of conditions such as diabetes and heart disease was found by the scientists.