Genes hold the information to build and maintain an organism’s cells and pass genetic traits to offspring, although some organelles (e.g. mitochondria) are self-replicating and are not coded for by the organism’s DNA.
The notion of a gene is evolving with the science of genetics, which began when Gregor Mendel noticed that biological variations are inherited from parent organisms as specific, discrete traits.
The biological entity responsible for defining traits was later termed a ”gene”, but the biological basis for inheritance remained unknown until DNA was identified as the genetic material in the 1940s.
All organisms have many genes corresponding to many different biological traits, some of which are immediately visible, such as eye color or number of limbs, and some of which are not, such as blood type or increased risk for specific diseases, or the thousands of basic biochemical processes that comprise life.
The vast majority of living organisms encode their genes in long strands of DNA. DNA (deoxyribonucleic acid) consists of a chain made from four types of nucleotide subunits, each composed of: a five-carbon sugar (2′-deoxyribose), a phosphate group, and one of the four bases adenine, cytosine, guanine, and thymine.