Genetics is the branch of biology that studies genes, genetic variation, and heredity in living organisms. It seeks to understand how traits are passed from parents to offspring and how these traits are expressed in individuals. The fundamental unit of genetics is the gene, a segment of DNA that contains the instructions for making proteins, which perform most life functions and form the structure of cells and tissues. DNA, or deoxyribonucleic acid, is composed of nucleotides arranged in a double helix structure.