How is skin colour controlled by genes?

Kat - Now it's time to look at your burning genetics questions, with the help of Naked Scientist Martha Henriques.

Martha - Listener Catherine Wells wanted to know about the genetics underlying differences in human skin colour. Why is it that children often have an intermediate appearance between their two parents? Mircea Iliescu from Cambridge University explained.

Mircea - Skin pigmentation is given by melanin, so melanin is a pigment which is in the skin cells. Melanin is produced in these granules in the cells which are melanosomes and this is quite a complex process of probably more than a hundred proteins and genes that are involved in producing this pigment. And so, the pigmentation eventually depends on the type of melanin, on the amount of these granules of melanin, and on the distribution of the granules in the skin. All these affects how dark or light the skin would be. These days, we measure for example with a melanin index, so we have a reflectometer machine which can measure skin reflectance - so, how light or dark the skin is and it gives us some melanin index of value. For example, the range can be let's say, from 25 to 100 where 25 would be a European skin and 100 would be someone in equatorial Africa. So, because we have a complex process and because there's a high range of variations, it is highly unlikely that one gene only, one change in one gene would cause all these difference like it happens with eye colour from blue to brown, and changes in several genes can alter the pigmentation values. So, just to give you an example, the gene which probably has the most effect is slc2485 which has an effect of about 10, 15 melanin index units. That's the most one gene can impact pigmentation and that is usually in an additive manner. So, if you have two copies of this gene, one change would maybe give it - create a difference of 5 melanin index units and the second copy, as it changes, gives you another 5 or 6. So, it's not really dominant like it happens with blue and brown eyes where if you have only then one copy of the brown gene let's call it, you'll get instantly brown eyes. And so, this process, there's a combinatorial effect of all these genes, each with their own effect and depending on what the parents have then the kid will have an intermediate range and it could have darker or lighter skin. That's how basically the range of pigmentation in the world is created.

Martha - So, a large number of genes contribute to human skin colour in a number of different ways which results in a subtle and complicated process influencing the child's skin colour. Thanks to Mircea Iliescu for answering that question from Catherine Wells.

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