DNA is the genetic material found in the nucleus of nearly every cell. A gene is a stretch of DNA that carries a set of instructions on how a protein should be made. These proteins carry out the functions of the body. Scientists estimate that humans have about 25,000 different genes. For example, there are genes that control eye color, genes that make proteins to break down food in the stomach, and genes that encode enzymes that regulate how cells grow. 

When the DNA of a gene is altered, a mutation is said to have occurred. Some mutations have little effect on how the body functions. Others are more serious, causing the production of defective proteins that result in disease symptoms.