how does environmental factors affect gene expression ,give me some examples related to environmental factors on gene expression

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.