Identical twins not so identical!

Identical -- or monozygotic -- twins come from a single fertilised egg that splits in two.

They are important research subjects because they are thought to have minimal genetic differences.

This means that when physical or behavioural differences emerge, environmental factors are presumed to be the likely cause.  

But the new research, published in the journal Nature Genetics, suggests that the role of genetic factors in shaping these differences has been underestimated.

Kari Stefansson, head of Iceland's deCODE genetics, and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children in order to track genetic mutations.

They measured mutations that occur during embryonic growth and found that identical twins differ by an average of 5.2 early developmental mutations.

In 15 percent of twins, the number of diverging mutations is higher.

When a mutation happens in the first few weeks of embryonic development, it would be expected to be widespread both in an individual's cells and in those of their offspring.