A person’s individual characteristics are due to both genetic factors (DNA) and environmental factors.

Genetic factors are due to small differences in the DNA between individuals. This is called genetic variation and is important for evolution because it provides a diverse set of characteristics that can be passed on through natural selection.

Everyone inherits their DNA (genes) from their parents, and some traits are due solely to this inherited DNA. These include genetic diseases (Huntington’s disease or sickle cell anemia), eye color, hair color, and skin color. Many other traits are due to environmental factors such as diet, hormones, or exposure to toxins. Even identical twins -- who have the same genetic make-up -- have different temperaments and personalities, so genes do not specify everything.

Allergies are an interesting case where both the environment and genetics are important. If your parents are allergic to something, then it is more likely that you will inherit the same allergy, so there is a genetic component to allergies. However, there is also an environmental component to allergies that is not well-understood. Human have evolved a complex immune system that recognizes potentially harmful agents like viruses and bacteria, and eliminates them from the body. The immune system learns to recognize a harmful substance like a virus, and then it stores a “memory” of this substance so that it can recognize it later. In allergies, the immune system mistakenly learns to recognize a normally harmless substance like a peanut protein or a grass protein as a harmful substance. When exposed to this harmless substance, the immune system recognizes it as harmful and launches an immune response to fight a non-existent threat. There is a hypothesis that not being exposed to enough germs and allergens as a baby makes it harder for the immune system to correctly learn what substances are OK and what substances are harmful, leading to more allergies in modern times as compared to the past, but no one knows for sure why some people get allergies and others do not. There is a lot of research being done by scientists who work on the immune system to figure out this mystery, and the answer is probably a combination of genetics (DNA) and environment.