Genes are the basic physical and functional unit of heredity. They are made up of DNA and contain instructions that tell your cells to make molecules called proteins. Proteins perform various functions in your body to keep you healthy, such as making eye pigments, powering muscles, and attacking invading bacteria. Each gene carries instructions that determine your features, such as eye color, hair color, and height. There are different versions of genes for each feature, and these are called variants. 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, and these small differences contribute to each person’s unique physical features.

Genes are found on tiny structures called chromosomes, which are found inside cells. Each gene has a special job to do, and the DNA in a gene spells out specific instructions for making proteins in the cell. Proteins are the building blocks for everything in your body, and they help your body grow, work properly, and stay healthy. Scientists estimate that each gene in the body may make as many as 10 different proteins.

The journey from gene to protein is complex and tightly controlled within each cell, and it consists of two major steps: transcription and translation. Transcription is the process of copying a DNA sequence into an RNA sequence, and translation is the process of using the RNA sequence to make a protein. Each cell expresses only a fraction of its genes at any given time, and the rest of the genes are turned off. The process of turning genes on and off is known as gene regulation, and it is an important part of normal development. Gene regulation can occur at any point during gene expression, but most commonly occurs at the level of transcription.