The fundamental unit of heredity in all living organisms is the gene. Genes are segments of DNA (deoxyribonucleic acid) that encode the information necessary for the development, functioning, growth, and reproduction of organisms. They determine specific traits by directing the synthesis of proteins, which are essential for various cellular processes. Understanding genes and their functions is critical for fields like genetics, evolutionary biology, and medicine as they provide insights into heredity, variation, and disease mechanisms.