gene

Scientists are not sure what causes MND, but it is likely to be a combination of the genes - or biological traits - you get from your parents when you are born, and other lifestyle factors.

When Liv Heeney was 28, she discovered she had a faulty gene that would probably cause her to develop a rare form of dementia at a young age.

“I think it’s the clutch gene,” said Dodger manager Dave Roberts.

"We achieved something that was thought to be impossible," said Prof Shoukhrat Mitalipov, the director of the Oregon Health and Science University's centre for embryonic cell and gene therapy.

She added: "Winter mortality of lambs removes inbred individuals, removes bad genes, so the flock has this purging thing the whole time which I don't think we would be able to organise."