Complex Traits

Genes are not the only things that can affect a complex trait. Often environmental factors can also be involved. The type of environmental factors can be very different for different traits. One obvious example of this is lung cancer. Smoking cigarettes greatly increases the risk of developing lung cancer. Smoking also seems to have an effect on other diseases, including some eye diseases (such as age-related macular degeneration). However, not every chronic smoker will develop lung cancer or eye disease: The presence of particular alleles of susceptibility genes is also a risk factor, as discussed below.

Figure 2. Here, the risk of developing a disease is either 0 or 9. It is determined by having at least one T at both Gene 1 and Gene 2.

Other environmental factors, which may be more difficult to identify directly, can still be identified by measuring other risk factors, such as gender, age, occupation, level of education, use of alcohol, and other measures of lifestyle. For example, the effect of estrogen may not be measured directly, but could be identified by finding an effect of gender on the risk of developing the disease. The strength of an environmental effect may vary by how much exposure a person has to that effect. For example, the number of years someone has smoked or how many cigarettes per day someone smokes may change the effect of exposure to smoke.