What causes freckles? And why do some people have them while others don’t?
• A freckle corresponds to a higher concentration of the pigment melanin, and is most obvious when it contrasts with fair-coloured skin. Freckles are associated with variants of the gene on chromosome 16 for the melanocortin-1 receptor (MC1R), which are also responsible for red or ginger hair. This probably explains why there is a correlation between freckles and red hair.
Melanocytes in the skin produce melanin and package it into called . These are passed into overlying keratinocytes, the cells that form the outer barrier of our skin, where they release their payload of melanin. Those born with darker skin have larger melanocytes, which lead to more melanin in the outer skin cells. Freckles are also associated with bigger melanocytes.
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Freckles are triggered by exposure to sunlight. UVB radiation activates melanocytes to increase melanin production, which can cause freckles to darken, increasing effectiveness as a sunscreen. The person tans relatively quickly where they have freckles, but the skin between is still prone to burning.
Red hair and freckles occur most frequently in people with northern or western European ancestry. For example, 13 per cent of Scots are redheads and about 40 per cent of them carry the red-hair gene.
Fair skin and freckles might bestow an evolutionary advantage to those living at high latitudes, where it is colder and the intensity of sunlight is lower. It is suggested that a paler complexion reduces heat loss through radiation, though clothing would surely be more effective at retaining heat.
The lighter skin pigmentation between freckles also leads to greater absorption of sunlight and higher production of vitamin D, reducing the incidence of rickets in northern latitudes.
Mike Follows, Willenhall, West Midlands, UK