Blue eyes and red hair
The genetic basis of blue eyes and the classic Celtic look – red hair and pale skin – has been discovered. Both are linked to the production of melanin
Although eye colour used to be considered a simple Mendelian recessive trait, the genetics of human eye colour are surprisingly complex. Eye colour depends on pigments in the iris (principally eumelanin), and many subtle shades exist. A key factor seems to be variation in the OCA2 gene. Certain mutations of this gene result in a form of albinism.
This is not just of cosmetic interest. Variation in OCA2 also affects freckling and skin pigmentation and is a risk factor for skin cancer.
Surprisingly, blue eyes result not from changes in OCA2 but in a nearby gene, HERC2, which regulates OCA2. In January 2008 several groups identified HERC2 variations – in fact, all present-day examples of blue eyes may have their origins in a single change that occurred 6,000–10,000 years ago, during the expansion of humans in the Stone Age.
Why did it persist? Blue eyes seem to offer no selective advantage. Perhaps it was chance. Or perhaps blue eyes were particularly attractive to Stone Age women?
Low melanin levels are also a feature of the classic ‘Celtic’ look – red hair and pale skin. The look is a feature of people with two inactive alleles of the gene for the melanocortin 1 receptor (MC1R). People with these genes need to be particularly careful about sun exposure, as they are more vulnerable to UV radiation and are at an increased risk of contracting skin cancer.
MC1R codes for receptors found on pigment cells, melanocytes, which make skin pigments – eumelanin and the lighter phaeomelanin. Variation in MC1R affects the ratio of eu- to phaeomelanin and hence the depth of colour in the skin. Redheads produce almost no eumelanin.
Interestingly, analysis of two sets of Neanderthal remains revealed variation in their MC1R gene sequences, suggesting that Neanderthals too showed variation in hair and skin colour pigmentation.
If the MC1R redhead allele is associated with such risks, why does it persist in the population? Possibly, at northern latitudes selection for dark skins has been so low that MC1R could mutate without harm; in earlier times people would not have lived long enough for the higher rate of skin cancer to be an issue. Some researchers have suggested sexual selection might be at play – redheads could have been highly sought after.
If you’re interested in finding out more about the genetics of physical appearance, see our article Genes that affect how we look.