Freckles are clusters of concentrated melanin that are most often visible with a fair skin complexion. Freckles are predominantly found on the face, although they may appear on any skin exposed to the sun, such as the shoulders.
They are an inherited trait, caused by variation in the MC1R gene that is involved in the determination of red hair, fair skin, and freckles. Freckle formation is triggered through exposure to sunlight.
Two types of melanin, the red pheomelanin and the black eumelanin, are present in human skin. Eumelanin protects against sunlight. People with red hair have mostly pheomelanin in their hair and skin and/or a reduced ability to produce eumelanin. This may explain why they fail to tan and are at risk from the ultraviolet radiation in sunlight.
- OMIM (Online Mendelian Inheritance in Man) 
- Valverde P. et al. 1995. Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nature Genet. 11: 328-330