The condition killed his sister Jessica, 11, three years ago and the only long-term cure is a bone-marrow or stem-cell transplant from a sibling or other compatible donor.
Samantha and David had battled with their local NHS trust for IVF to have a "designer baby" whose umbilical cord would provide stem cells to save Alex. Heartbreakingly, the IVF cycle failed. But then, against the odds, Samantha naturally conceived healthy Ethan - who was a perfect match for sick Alex.
Samantha, 42, says: "He truly is a miracle. The angels must have been watching over us when he was conceived. We would have brought Ethan into the world regardless of whether he had FA, but if he can save his brother then that will be the most amazing thing."
The McDowells had launched their campaign for IVF in 2009 with the hope of creating a so-called "saviour sibling". Doctors would screen embryos to find one without the faulty FA gene and create a child without the disease, who would also be a tissue match for Alex.
After the longed-for NHS-funded IVF cycle failed, the couple could not afford to try again and feared Alex would eventually die from his condition, which can cause cancer and bone marrow failure.
But then, as it seemed all hope was lost, fate intervened when, astonishingly, Samantha fell pregnant naturally.
Now four months old and free of FA, he has been confirmed as a perfect tissue match for Alex.
David, 39, says: "Alex adores his baby brother but he's too young to understand the implications of all this. Ethan will be responsible for saving his brother's life and, as parents, that's more than we could ever have wished for."
It is a long wished-for happy ending for the McDowells after five years of heartbreak.
Their daughter Jessica was nine when she was diagnosed with Fanconi anaemia in 2006. Tests revealed that her parents, who live in Malton, North Yorkshire, were both carriers of the gene, which means their children have a one in four chance of being born with it. Their eldest Ashleigh, 16, does not have the condition but was not a close enough match to be a donor for Jessica or Alex.
David says: "We'd never even heard of FA before Jessica was diagnosed. It is a very rare condition that only affects three in a million people worldwide.
"We quickly realised it was very, very serious. Jess would die if she didn't have a bone marrow transplant. The best option would have been a 100 per cent tissue match from a sibling, but obviously we didn't have that option, so they had to use a 50 per cent match from me."
Jessica's transplant went ahead in May 2008 but tragically she died three months later from multiple organ failure after suffering complications. To add to the heartbreak, Samantha had found out a year earlier, when she was six months pregnant with Alex, that he had FA, too.
She says: "It was heartbreaking. Obviously we'd been here before and seen the pain and suffering it caused. But there was no way I could terminate my pregnancy. Despite her illness, Jess was the happiest little girl and we were lucky enough to spend 11 wonderful years with her. We knew we had to give our little boy a chance at life too. I'm just so glad Jess got a chance to meet her little brother before she passed away."
Desperate to help save Alex, the McDowells' only hope seemed to be a so-called "saviour sibling" for Alex. But they stumbled at the first hurdle when they found they were the victims of a cruel IVF postcode lottery.
Samantha says: "We fought our NHS trust for a year before they agreed to fund the IVF treatment. Even then, they only agreed to do one cycle. We felt really let down as, according to NHS guidelines, we should have been eligible for three cycles, but by that point I was approaching 40 and fell outside the age restrictions.
"But I wasn't doing this because I had poor fertility - we had to have screening as it was simply the only way we could guarantee our baby wouldn't have FA."
In March last year Samantha underwent the £6,000 treatment. She and David were devastated when it failed, fearing they had lost their only chance to save Alex. She says: "We desperately wanted another baby anyway, not simply as a 'saviour sibling', which is a term I don't like as it suggests they're only useful for one thing, but it wasn't to be. I suffer from severe arthritis and David is my full-time carer so unfortunately we were in no position to pay for IVF privately. The doctors had implied I was too old to get pregnant without it anyway, so we just tried to accept it and move on."
It was then that fate intervened. Four months later, last July, Samantha was shocked to find out she was pregnant.
She says: "When I saw the pregnancy test I couldn't believe it. It was so unexpected. I was partly thrilled and partly terrified. After all, IVF would have screened for the gene, but having conceived naturally, there was a one-in-four chance this baby would have FA. We tried to be optimistic but it was very hard having already seen two of our children suffer so much."
The couple took the difficult decision to not test for the condition during pregnancy.
Samantha says: "I wouldn't blame any parent for testing and subsequently choosing a termination but in our case it's not something we could go through with.
"That's why it upsets us when people said we just wanted a baby so he could be a saviour sibling. In fact, he was a much-longed-for child and we were going to give him a chance at life regardless of the outcome."
Thankfully, early scans showed the baby was developing normally - a small or underdeveloped foetus is often a tell-tale sign of FA. Samantha gave birth to Ethan by caesarean section in April, then had to wait an agonising eight weeks before doctors confirmed he did not have FA.
Samantha says: "I cried with happiness when we heard the news. It was such a relief. Ethan has since undergone tests and was found to be a perfect match for his brother. It's more than we could ever have hoped for.
"It's a double miracle as our baby could have been free of the disease but not a match. What is heartbreaking is that Ethan would also have been a match for Jessica were she still here, as tests have shown Alex and Jessica have exactly the same genetic fingerprint."
The couple hope Alex will be well enough for a bone-marrow transplant in the next few months, using stem cells from his brother's umbilical cord. And although Ethan is an "accidental" saviour sibling, his parents support families who use IVF for genetic screening.
David - who is studying a degree in molecular sciences in the hope of working in genetic research into FA - says: "We were very lucky Ethan wasn't born with FA. But IVF is the only way of making sure it doesn't happen.
"I know genetic screening is often controversial but how many parents could say they wouldn't do the same thing if they were in our shoes?
"A lot of people mistakenly believe the child's life will be dictated by his sick sibling and he'll spend his whole life being poked and prodded and cut up to provide 'spare parts'. But that couldn't be further from the truth.
"Ethan can save Alex using stem cells from his umbilical cord which otherwise would have been discarded. The doctors don't need to touch him at all. He's going to save his brother's life but he's blissfully unaware of it."
Samantha adds: "Thanks to Ethan, Alex is hopefully going to grow up to be a healthy and happy little boy, and when Ethan is old enough we're going to tell him how he saved his brother's life. We love him to bits - he truly is our little miracle."
DAVID and Samantha have set up a charity, Fanconi Anaemia Family Support. To donate visit www.fanconisupport.info
Via Mirror
