Experts and laypeople alike once thought that simply taking in more calories than are expended caused a person to become obese. However, scientists have determined that there are many factors—including genetics, the environment, and behavior—that contribute to obesity in varying degrees for different people.

The Role of Genetics

Genes are the part of a DNA molecule that pass hereditary information from parents to their offspring. They reside on wormlike bodies called chromosomes in the center, or nucleus, of each cell.

The sequence of genes on each chromosome provides the cell containing those chromosomes with a set of instructions on how to grow and operate. Humans have forty-six chromosomes in each cell. Twenty-three come from the mother and the other twenty-three from the father. The genes on each chromosome also come in pairs, with one copy of every gene from the...