Genetics test could, in principle, identify which identical twin fathered a child

A paternity dispute involving identical, monozygotic twins has sparked an argument that’s equal parts courtroom and laboratory bench. In a case covered on 30 April, a court of appeal said it could not rule on which twin fathered a child—stating it was “not possible” to say which twin was responsible.

But a letter published in response pushes back hard. The writer says the court’s framing is “definitely not true,” arguing that the germ cells of monozygotic twins differ with enough probability—and enough measurable degree—that their respective children can be assigned to one twin or the other. The method, the author explains, relies on molecular genetic techniques, the same kind of approach that has become more routine across genetic testing over the past decade.

The key claim is pretty specific: even though monozygotic twins begin life from the same fertilized egg and share essentially the same DNA at birth, there can still be differences between their germ cells over time. Those differences, the letter argues, show up strongly enough in the genetic record of offspring that a lab can “clearly assign” the child to either twin. And the author adds that teams can’t just do this in theory. According to the writer, ideas for this approach were presented in 2012, and practical feasibility was clearly demonstrated in 2018.

Still, reality has a price tag. The letter acknowledges that the required molecular genetic testing entails considerable costs—“currently in the five-figure range.” What’s more, it questions the court’s characterization that such costs would be “very significant” to the point of ruling out genetic testing. The writer suggests the consequences of doing nothing may be more serious for everyone involved than the expense of testing.

There’s also a human, everyday texture to the debate—even if the dispute is technical. Somewhere in the process, someone is sitting under fluorescent lighting waiting for results, maybe with a coffee that’s gone slightly cold, listening to the quiet whir of lab equipment in the background. That kind of mundane detail is hard to fit into legal language, but it helps explain why cost arguments matter: the people affected don’t just want an abstract principle, they want answers.

Misryoum newsroom analysis indicates the heart of the disagreement is less about whether DNA can sometimes be used to infer relationships, and more about whether this particular scenario—two genetically near-identical men, one child—can be resolved to the court’s standard of certainty. If the approach described in the letter can indeed reliably distinguish which twin fathered a child, then “not possible” sounds like it’s cutting off a pathway that some labs may view as available.

The letter ends on a pointed note: if testing is feasible, the threshold for deploying it—especially when paternity consequences are on the line—becomes the real question. And the broader story underneath is familiar in genetics: a tool that can exist in principle, and may even work in practice, can still run into institutional limits—cost, procedure, and the demanding idea of what courts consider provable.

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