To understand this study, we must explain a few key concepts of genetics. A gene is a unit of heredity, consisting of a string of DNA. Functionally, genes regulate the production of proteins. Because human chromosomes, which are strings of genetic material divided into genes, come in pairs, each gene has two copies. Alleles are variations of the same gene. Humans have two alleles of each gene. The genotype is the specific genetic makeup of an individual. The genotype combined with environmental factors produce observable characteristics called phenotypes, such as whether or not a person has blue eyes or is obese. An individual’s two alleles may be the same, but often differ. When two alleles differ, one may be dominant and one recessive, such as for eye or hair color. Or the phenotype may depend on the combined alleles in another manner, possibly also depending on the environment.