My sister is the only person in the family with ginger hair. My baffled family suggested her hair might darken with age but it hasn’t. My sister’s hair was a mystery until my first school genetics lesson, when we learned about classical Mendelian inheritance. According to Mendel, all genes have two copies and versions (or alleles) of genes can be either ‘dominant’ or ‘recessive’. So when an individual has two different versions of a gene, the dominant version dictates the physical characteristic; a recessive version only dictates the characteristic when both copies are recessive. Although Mendel’s rules apply to many characteristics, there are also many features determined by far more complex genetics, for example, a person’s height is determined by numerous genes related to growth.
Ginger hair is caused by a recessive gene version so my sister must have two copies of this, and she must have inherited one from each of my parents. Therefore, both of my brown-haired parents must have one ginger version and one brown version, but only the dominant brown version has affected their hair colour. With my brown hair, there is a 2/3 chance that I’ve inherited a ginger as well as a brown-haired version.
The gene that causes ginger hair makes a protein called melanocortin 1 receptor. This protein responds to hormone signals to help produce eumelanin, a dark pigment in skin and hair. The mutated ginger version of this gene does not respond to the hormone, producing red coloured pheomelanin instead of normal eumelanin. Those with only one copy of the mutated version can still produce plenty of eumelanin pigment in their hair, though they often have pale skin and, like red-heads, a tendency to burn instead of tan (my pallor leads me to believe I am of this camp). Those with two mutated versions produce an excess of pheomelanin and very little eumelanin, giving red hair and pale skin. Alternatively, albinism is caused by a complete lack of either kind of melanin.
Eumelanin is an important part of the skin’s defence against UV damage. We produce extra eumelanin when we tan in the summer as a temporary protection against the sun’s harmful rays. People living near the equator (or whose ancestors did) have evolved to have more eumelanin and so darker skin than those from more temperate climes. Without it, ginger-haired people burn easily without tanning. Sunburn hurts but it’s not the worst of it as UV radiation also causes DNA mutations which can cause skin cancer.
In our suncream happy, indoor working existence, skin cancer is less of a risk but in hunter-gatherer days, it could severely reduce your chances of having children and you’re not going to be much of a hunter if you spend all summer red and raw. Evolutionarily, you’re not doing so well. So how would an evolutionarily disadvantageous characteristic persist?
Red hair is most common amongst the Celtic populations from Scotland and Ireland. In ancient times, small tribes formed as they budded off larger tribes. When a small tribe buds off, there is a genetic bottleneck. The gene pool of the new tribe is much smaller than that of the larger tribe, particularly as the small tribe is likely to consist of related individuals. In this small gene pool, chance plays a more important role than in the larger gene pool. So by chance, a rare gene mutation might be common in the group that buds off. And by chance, unless a genetic mutation confers a disadvantage to the individual, a mutant version can increase in frequency in a population. This is called genetic drift. So the Celts, by chance, had a high frequency of the ginger mutation which was able to persist over time.
But hang on, didn’t I say that the ginger mutation causes sunburn and an increased risk of skin cancer? And that genetic drift only occurs when a gene doesn’t cause a disadvantage? I also said that the Celts were the predecessors of the Scots and the Irish – two populations who consider a square inch of blue sky to be a glorious day. When heat-waves are myths put about by the English, pale skin that burns easily is no disadvantage.
Both my parents likely have a Celtic ancestor or two and my sister’s ginger hair comes from her mutated genes for pigment manufacture, inherited from these distant relatives. Knowing all this, I can confidently bat off all the jokes about my mum having affairs with a red-haired milkman.