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How identical can fraternal twins be?

Asked by Lavanya P.2.9k views1 answer
AM
Arjun Mehta
PhD candidate in population genetics, IISc

Fraternal twins share, on average, 50% of their DNA - the same as any pair of siblings. But "on average" hides a wide range.

The actual figure for any specific pair of fraternal twins can range from roughly 38% to 61% shared DNA. This is the same as the range for non-twin siblings, but it gets noticed more in twins because we compare them more carefully.

So at the lower end of the range, fraternal twins can be about as similar to each other as second cousins. At the upper end, they can be almost as similar as half-identical twins (a rare phenomenon where one egg is fertilised by two sperm).

In appearance, fraternal twins can range from looking nothing alike to looking essentially identical at a glance. Same-sex fraternal twins who happen to draw genetically similar combinations sometimes get mistaken for identical twins their whole lives, and only discover the truth from a DNA test.

There is also a rare class - sesquizygotic or "half-identical" twins - where the genetics are between fraternal and identical. Fewer than 10 cases have been documented worldwide. So if your fraternal twins look uncannily alike, the explanation is almost certainly the high-end-of-normal random draw, not the once-in-a-decade special case.

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