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The expression of genes in an organism can be influenced by the environment, including the external world in which the organism is located or develops, as well as the organism's internal world, which includes such factors as its hormones and metabolism. One major internal environmental influence that affects gene expression is gender, as is the case with sex-influenced and sex-limited traits. Similarly, drugs, chemicals, temperature, and light are among the external environmental factors that can determine which genes are turned on and off, thereby influencing the way an organism develops and functions.
Skin color is a polygenic trait, meaning more than one pair of genes determines skin color. However, it is multifactorial as well because it has a strong environmental component in the expression of the person's phenotype for skin color. If someone is in the sun very often, perhaps in a climate closer to the equator, their skin color will be affected due to more melanin production, as compared to someone in a more northern climate with less direct insolation. Height as another genetic trait that is polygenic and multifactorial. Factors in the environment such as proper nutrition, exercise and not smoking all affect one's height. You can be born with a potential to be a certain height, but your environment will influence genetic expression also. Multifactorial traits follow a bell-shaped curve in their distribution, with most of the population around a median value and with less of the population at either extreme. Another example would be the autoimmune disease diabetes, which is a genetic trait. If a person has the genetic potential to develop diabetes, their level of exercise, their food choices and their body weight will definitely affect gene expression for this disorder. Diet, exercise, weight, even exposure to a virus are all environmental factors that play a role in triggering this genetic condition.
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