Sisters Annemarie Atha and Geraldine Rowing became the first identical twins to undergo a live liver transplant in the UK.
But the sisters did not even realise they were identical until one of them needed the surgery.
Ms Atha, 48, said she did not think twice about donating part of her liver to her twin sister Geraldine Rowing, after her doctor recommended a transplant.
Ms Rowing’s liver had been damaged by cancer a few years earlier.
Ms Atha said: “It was an added bonus to find out that, despite everything we had always been told by our mother, we were genetically identical.”
Finding the perfect donor, so close to home, meant her sister did not need to take a cocktail of drugs, which are usually mandatory, to stop her body rejecting the new organ.
Ms Rowing, said: “I would just like to say thank you very much Annemarie, from the bottom of my heart.”
She mentioned to her doctor that she was a non-identical twin and he suggested doing tests - which revealed the sisters were genetically identical, despite not looking exactly the same.
Ms Atha said: “Mum always said that because there were two placentas when we were born that we weren’t identical, so we’ve grown up believing we were not.
“But the doctors said ‘you’re identical enough to do the transplant’.”
The sisters, from Rothwell, underwent the operation in April and Ms Rowing said she is now feeling much better.
She said: “I’m feeling better by the day,”
Ms Rowing said not having to take immuno-suppressant drugs has made a real difference to her.
Consultant liver surgeon Raj Prasad, who performed the operation, said live liver transplants - where a living person donates part of their organ for the operation - are becoming more common.
He said the benefits of identical twins having the operation was massive because Ms Rowing didn’t have to take the additional drugs, which would have increased her risk of further cancer.
He said: “It’s an absolutely massive advantage which is God or nature’s gift. It’s a dream.”