Identical twins, more properly known as monozygotic twins, are the result of a single fertilized egg splitting into two embryos. Consequently, it is assumed their genetics are the same, and any differences within a pair must be environmental. A new study has found thi is less accurate than thought, with some twins having a substantial number of mutations not shared by their counterparts. Considering how much the “nature versus nurture” debate has been driven by thousands of twin studies, this could be more than a scientific curiosity.

Mutations that occur when cells divide drove our evolution from the first organisms, so naturally twins have some, many of which appear after embryos' separation. However, most comparisons between monozygotic and dizygotic (fraternal) twins have treated divergence from mutations as too minor to matter.

Dr Hákon Jónsson of Iceland's deCODE genetics sought to investigate mutation rates by sequencing the genomes from both members of 387 pairs of monozygotic twins, as well as their parents, children and even spouses. On average, Jónsson and co-authors report in Nature Genetics, each pair of twins have 5.2 early developmental mutations that occur in one but not both of them, along with a larger number that appeared later in life. Many of these mutations will be in genes where the effects are hard to detect. Others are not, and may cause some of the differences previously attributed to the environment. The paper mentions autism as one condition sometimes seen in only one monozygotic twin that may result from mutation rather than environment.

More intriguingly, the authors found in 15 percent of pairs the genome differences are substantially more frequent. Rather than the number of twin-specific mutations following a bell curve, this subset had more than 100 that only appeared in one twin, far beyond the rest, greatly increasing the chance some will produce noticeable differences.

The findings also shed light on the timing at which mutations appear, since anything happening before separation would be shared by both twins.

Many monozygotic pairs follow very similar life paths, such as newly elected US senator Mark Kelly and Scott Kelly, who both became astronauts, providing NASA with an excellent opportunity to study the effects of prolonged weightlessness on the body with one twin in space and one on Earth. Nevertheless, some twins surprise with their differences, either in personality or talents. This is usually attributed to environment, perhaps reflecting a deliberate desire on the siblings' part to distinguish themselves. However, Jónsson and co-authors' new research suggests the contribution of mutations during early development has been underestimated.

If so, many studies may require some reanalysis. It is common to see how much monozygotic twins differ on measures such as intelligence or sexuality and compare this with the differences in dizygotic twins. If the monozygotic gap is entirely environmental, the reasoning goes, then the extent to which the dizygotic variation is larger signifies the contribution from genetics. However, if monozygotic differences have a mutation-driven genetic component as well it may complicate this apparently straight-forward test.