Differences in human skin color are determined by different amounts and types of several pigments, collectively called melanin, which are produced by skin cells. Skin color is a complex trait. Unlike simpler, single-gene traits, complex traits arise from the contributions of many genes, as well as from environmental factors.
In human populations, many genes with multiple alleles (genetic variants) affect skin color. Scientists who study skin color often use instruments called spectrophotometers to measure how much light is reflected off the skin of the inside upper arm or the buttocks. (Anything for science, right?) Since these areas of the body generally receive very little sunlight, they represent baseline skin color, which is largely under genetic control.
You may have noticed that the color of the skin on your forearm differs from the color of the skin on the inside of your upper arm. When skin is exposed to sunlight, the ultraviolet (UV) wavelengths of light may induce skin cells to produce more dark-pigmented melanin, making the skin darker. Melanin can absorb UV light, and thus helps protect molecules in skin cells from damage by the sun.
Many factors affect how much sun your forearm gets which, in turn, affects how much extra melanin the skin there produces. What time of year is it? What’s the weather like? Do you live in a sunny or a shady place? Do you usually wear long or short sleeves outside? Do you wear sunscreen on your arms every day, or not? Do you spend a lot of time outside, or only a little?
Comparing your results to others, you may notice that some people tan more than others—that is, there’s more of a difference between their inner arm and their outer arm. There can be many reasons for this—some are genetic; some are environmental. Examples of environmental effects might include your hobbies (if you spend most weekends outside playing soccer, for instance, while someone else paints indoors), your culture (if, for instance, yours favors wearing clothing that covers the arms), or your job (an office worker, for example, might spend less time in the sun than a construction worker).
A lack of visible tanning on the forearm can also be due to genetic factors. Some light-skinned people get sunburned rather than tanned by the sun; for genetic reasons, their skin does not respond to UV light by producing more melanin. Some people whose skin is very dark may produce more melanin in sunlight, but it may not be noticeable. In this way, skin color is an interaction between genes and the environment, with people of different genotypes responding differently to the same environmental conditions.