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In dominant disorders, like Huntington’s, a child needs to inherit only one mutated gene to develop the disease, so one homozygous parent, also a rarity, inevitably would pass on the disease to all embryos.

With help from a Moscow hearing clinic, this summer Rebrikov identified five couples who are homozygous for 35delG and certain to have a deaf child.

To their surprise, in August, they learned they are both also 35delG homozygous, indicating their daughter had inherited the mutations, rather than having new ones.

He later found that CCR5Δ32 homozygous humans who get infected with that virus are more susceptible to severe encephalitis and even death than other people.

If follow-up trials in monkeys work as well, the scientists will launch a trial in patients with homozygous familial hypercholesterolaemia, or HoFH.