A
gene is a molecular unit of
heredity of a living
organism. It is a name given to some stretches of
DNA and
RNA that code for a
polypeptide or for an
RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's
cells and pass genetic
traits to offspring, although some
organelles are self-replicating and are not coded for by the organism's DNA. All organisms have many genes corresponding to various 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.