A gene is a segment of DNA that codes for a specific protein. Proteins are the building blocks of the human body and perform many vital functions. Genes are passed down from parents to offspring and are responsible for the traits inherited. Mutations, or changes in the DNA, can occur in genes and can be passed down to future generations. Mutations can be beneficial, harmful, or have no effect on the organism. An international research effort called the Human Genome Project, which worked to determine the sequence of the human genome and identify the genes that it contains, estimated that humans have between 20,000 and 25,000 genes.

For example, if both of your parents have green eyes, you might inherit the trait of green eyes from them. Or if your mom has freckles, you might have freckles too because you inherited the trait for freckles. Genes aren’t just found in humans, animals and plants contain genes as well.

Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

Scientists keep track of genes by giving them unique names. Because gene names can be long, genes are also assigned symbols, which are short combinations of letters (and sometimes numbers) that represent an abbreviated version of the gene name.