However, the truth is always complicated. It isn't so simple as if you have the gene for a disease, you will get the disease.. Here are some of the reasons why: 1  While genes play a role in many disorders, so do the conditions and circumstances of your life and the decisions you make.2  In other words, heredity may influence your health, but so does your environment. 3  For example, some people have genes that put them at risk for cancer. Only a few human diseases are triggered by a single gene working by itself. 2  In most cases, a disease results from the actions of many genes. 3  An example of a single-gene disorder is Huntington's disease, which was discussed in the last chapter. 4  One error in one gene leads to the fatal health problems of HD.