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A girl resembling Madeleine McCann has been spotted in France by a woman who says she had the same eye blemish as the missing girl's. Police have dismissed the sighting, but how common is the defect?
Called a coloboma, the defect can appear as a keyhole shape
Referred to as the "mark of Madeleine" the blemish on the four-year-old's right eye has played a key part in the campaign to highlight her disappearance, emphasised in posters and videos.
It was at the centre of the latest possible sighting of her. Dutch student Melissa Fiering says she saw a girl who looked like Madeleine McCann at a service station in the South of France.
She believed it was the youngster, who went missing from the Portuguese resort of Praia da Luz last May, because she had a dark smudge on her right iris. However, police examining CCTV evidence have determined it was not the missing girl.
Colobomas are extremely rare - about 0.007% of the population
It's not the first reported sighting of Madeleine, but a spokesman for the McCann family said it was "more worthy than most" because of the eye defect.
The blemish is called a coloboma of the iris. Its precise incidence is not recorded in the UK, but figures suggest it occurs in about 0.007% of the population. A 1989 study in China found two cases in a survey of 26,512 children under 12 years of age. That would mean a prevalence of 7.5 per 100,000.
In the UK, cases of coloboma are lumped in with two other congenital eye defects - microphthalmia and anophthalmia - says the Micro and Anophthalmic Children's Society (Macs).
"There are about three to seven babies every 100,000 live births, born with any one of these defects," says a spokeswoman for the society, although "coloboma is more common than the other two conditions."
A coloboma occurs when the eye fails to develop fully while the baby is in the womb. It leaves part of the structure missing, creating a gap.
Colobomas can occur in several parts of the eye - the word itself means an absence or defect of tissue. When a coloboma is in the iris - like Madeleine's - it can give the appearance of a keyhole. In very rare cases a person can get one in each eye.
"They're pretty uncommon," says Iain Anderson, chairman of the Eyecare Trust. "In my patient base of 5,000 just one person has one. It is a significant distinguishing feature in identifying someone."
It's not known why colobomas occur but there appears to be a strong hereditary factor.
Having a coloboma can cause sight problems but it depends on which part of the eye is affected and to what extent, according to Dr Trisha Macnair.
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"If the iris is affected, the pupil may be oval or keyhole-shaped rather than round, and central vision may be affected," she says.
While French police have ruled out the latest "sighting" of Madeleine, given the rareness of the eye defect it's perhaps not surprising Ms Fiering believed she had seen the missing girl.
"I looked in her eye because I'd seen on TV that Madeleine's got a defect in her right iris and I saw this girl had it," she told the Daily Mirror. "I could hardly believe my eyes, but I knew for sure it was her."