Genetics plays a powerful role in human development. Genes determine an organism's characteristics such as eye color, height, skin color, handedness, and a myriad of other traits that make up a human being. However, genes do not act in a vaccuum. The environment plays a role in genetic expression. For instance, a person might have the genetic potential to develop multiple sclerosis but it may not occur until an environmental trigger like a virus causes this condition to be expressed during a person's lifetime. The same can be said for diabetes and other autoimmune diseases. A person may have the genetic "potential" to be tall however, if they follow a poor diet, and perhaps smoke, the environment will affect gene expression. Although genetics plays a very powerful role in how a human develops, one cannot downplay the role of the environment on gene expression.