How is genetic variation related to disease?
Nearly all human diseases are influenced by genes. Because individuals have different variants of genes, it follows that the risk of developing various diseases will also differ among individuals. Consider a simple example. Jim Fixx, a well-known runner and fitness enthusiast, died of a heart attack at the age of 52. Sir Winston Churchill, who was renowned for his abhorrence of exercise and his love of food, drink, and tobacco, lived to the age of 90. It is plausible that genetic differences between Fixx and Churchill were responsible, at least in part, for the paradoxical difference in their life spans. (Indeed, Jim Fixx’s father had a heart attack at the age of 35 and died of another heart attack at the age of 43.) Because genes are passed down from parents to offspring, diseases tend to “cluster” in families. For example, if an individual has had a heart attack, the risk that his or her close relatives offspring or siblings will have a heart attack is two to three times higher than
