Both Friesian cows and zebras are black and white. Does that mean both animals have identical code in their DNA that specifies 鈥渂lack and white鈥�?
鈥� Not really, although they may well have the same code for dark pigments, specifically a few types of melanin. Melanins have been in our ancestors鈥� metabolic repertoire for well over 300 million years, and vertebrate bodies use them in many ways.
White is different; it results not from pigment but from the lack of it. (It also makes use of structures that diffuse light, much as snow does.) Hairs and feathers produce no melanin: special cells called melanocytes inject pigment into the cells of growing hair.
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An animal鈥檚 colour pattern depends on the distribution and activity of these melanocytes. Even in individuals that can produce intensely black melanin, areas without melanocytes will be white. Albinism arises from a genetic make-up that cannot produce melanin, but this is not the explanation for the white we see in Friesians or zebras.
Instead, the patterning is down to the distribution of melanocytes in the embryo. These cells originate in the embryonic neural tube along the spine, then migrate generally down the flanks of the embryo in patterns that vary with the species, as in zebras, or with individual genetic make-up, as in Friesians and many domestic pets. In some cases, temperature effects can also alter melanocyte activity 鈥� this explains the distinctive colouration of Siamese cats.
Jon Richfield, Somerset West, South Africa
鈥� Hair colour is determined by a type of pigment-making cell found in the hair follicle. These cells can make different types of melanin, determining the colour and shade of the hair. Enzymes, coded for in the genome, make the pigment.
鈥淥ne type of melanin is concentrated in our lips, nipples and genitals, giving the pinkish colour鈥�
One type of melanin is especially concentrated in the lips, nipples and genitals of humans, giving the pinkish colour. Many animals have dark skin, which is down to a similar pigment as in dark hairs, with similar genes involved. The actual pattern of the colouring is generated by sets of genes that cause different areas to turn on different processes.
Neil Shirtcliffe, Professor of Biomaterials, Rhine-Waal University of Applied Sciences, Kleve, Germany
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