As girls, they insisted on matching outfits, right down to the ribbons in their blonde pigtails. And as teenagers, identical twins Gwennan and Elin Thomas were inseparable, even going to the same university.

‘We were, and are, best friends,’ says Gwennan. ‘I had a freckle on my forehead as a baby – which disappeared – and that was how our parents told us apart. Looking at some childhood photos, we still argue about who is who.’ Yet, in 2002, a shocking, inexplicable event would mark the sisters apart.

That year, Gwennan was 25 and diagnosed with alopecia. It is a condition in whixh hair losses occur even from eyebrows and eyelashes. Ten years on, she can still remember the morning she woke to find clumps of hair on her pillow. ‘I burst into tears,’ she recalls. ‘My hair had been thinning for a while but then it started falling out in clumps pretty much overnight.’

What makes Gwennan’s experience extraordinary is that her identical twin sister Elin remains completely unaffected, even though the sisters share exactly the same genes.

Experts believe theirs and a handful of similar cases, are proof that environmental factors are a major risk in developing the condition and that this discovery will pave the way for a new understanding of this devastating, difficult-to-treat problem.

‘In identical twins with alopecia, about 50 per cent both have the condition. That means roughly half of the contributing causes of alopecia are non-genetic and may be environmental,’ says Dr Angela Christiano, a world leader in the study of alopecia based at Columbia University Medical Center.

 No difference: Gwennan and Elin, aged seven, eating an ice cream with their brother William

‘Losing my eyebrows and eyelashes was even more traumatic as those define your facial features. And it made it worse having a sister who was the mirror image of me , but with hair. Of course I didn’t want Elin to suffer too, but every time I looked at her I was reminded of what I’d lost.’

Elin remains mindful that, given the genetic link, she too could well experience alopecia. ‘I’m prepared for that,’ she says.
‘I’ve seen how amazingly Gwennan has coped and she’s a real inspiration. She’s living proof that you can have alopecia and still hold your head up high.’