Identical twins may not be carbon copies of themselves at the DNA level, a new research has ruled.


Before now, identical twins have often been described as “either of a pair of twins who, as a result of developing from a single fertilized ovum, are alike in all genetic characteristics and typically very similar in appearance.”

They were thought to be the same and health differences were considered to be the product of their environment.

But a new study published on Thursday on Nature Genetics found that identical twins differ by 5.2 genetic changes on average.


For the study, Iceland researchers deciphered the genetic makeup, or genome, of 381 pairs of identical twins.

Of those, 38 pairs were genetic duplicates of each other, but most had some differences in DNA that are believed to have arisen very early in development, either just before one embryo split to form two or shortly after the split.

Some twins had many genetic differences, including 39 pairs who had more than 100 changes between the twins.


The study ruled that mutation patterns among twins suggest that embryos don’t split neatly when the twins form. The researchers said some twins may arise when a single cell or a small group of cells splits off from the embryo.

According to the scientists, the number of cells that a twin originates from may determine how genetically different they are from their twin, with more uneven embryo splits leading to a greater number of differences between the twins.

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