There’s an incredible range of skin and hair color in people from around the world. And it’s caused almost solely by a single pigment called melanin. How does one color do it all?
How is it possible to make all of the colors of the human body out of the single pigment melanin?
Melanin is what causes the spectrum of colors in human skin and hair. The trick is that melanin isn’t just a single color. Depending on its chemistry, melanin comes in two different forms, one that varies from black to brown, and another that ranges from reddish-brown to yellow.
The cells that produce melanin make a genetically determined ratio of the different colors. For example, a typical Nigerian might produce a large amount of mostly darker melanin. Meanwhile someone from Ireland might produce much less melanin, mostly of the reddish form. And there’s a whole spectrum of color combinations in between.
Skin color is also influenced by the way melanin is distributed in a skin cell. When it’s produced, melanin is packaged in tiny sacs or granules. If the granules are large and dispersed evenly throughout the cell, the pigment covers a greater area, and the skin appears darker.
If the melanin granules are small, and clustered together in the skin cell, the pigment covers a smaller area, and the skin appears lighter.
Melanins fall into two groups: the eumelanins, which come in shades of brown and black, and pheomelanins, which come in red and yellow forms. Scientists have been unable to elucidate the chemical and molecular differences between the different forms because they are extremely difficult to purify.
Melanin is produced in the skin, the hair follicles, the eyes, ears and brain. Scientists don’t know what the function of melanin is in the ears and brain, but they know melanin is important for sun protection in the skin and for eye development.
Along with skin and hair color, melanin is also largely responsible for the range of eye color in humans. Black- and brown-eyed people have more melanin, closer to the surface of the iris, where it absorbs light. People with lighter eyes, like green and gray, have less melanin, positioned toward the back of the eye, where less light is absorbed and more is reflected.
One person in 17,000 in the USA has some type of albinism. Because it is a recessive gene, the parents of a child with albinism usually have normal hair and eye color for their ethnic background, and often no history of albinism in their family. Classic albinism is caused by a gene that makes the enzyme tyrosinase defective. Tyrosinase transforms an amino acid, tyrosine, to the pigment melanin, so as a result, these individuals are unable to make any pigment. There are several other genes that cause different types of albinism. Some individuals with albinism have normal skin and hair, but lack melanin in the eyes. Melanin is important in the development of the retina and in the development of the nerve connections between the retina and the brain. For this reason, all individuals with albinism have trouble with their vision.
Bibliography: “Human pigmentation genetics: the difference is only skin deep” Sturm, R.A., N.F. Box and M. Ramsay. 1998 Review article, BioEssays 20:712-721.