In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce RNA. There are two types of molecular genes: protein-coding genes and non-coding genes. During gene expression (the synthesis of RNA or protein from a gene), DNA is first copied into RNA. RNA can be directly functional or be the intermediate template for the synthesis of a protein.

Chromosome (107 - 1010 bp) DNA Gene (103 - 106 bp ) Function A chromosome and its packaged long strand of DNA unraveled. The DNA's base pairs encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.

The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits from one generation to the next. These genes make up different DNA sequences, together called a genotype, that is specific to every given individual, within the gene pool of the population of a given species. The genotype, along with environmental and developmental factors, ultimately determines the phenotype of the individual.

Some biological traits occur under the combined influence of polygenes (a set of different genes) and gene–environment interactions. Some genetic traits are instantly visible, such as eye color or the number of limbs, others are not, such as blood type, the risk for specific diseases, or the thousands of basic biochemical processes that constitute life. A gene can acquire mutations in its sequence, leading to different variants, known as alleles, in the population. These alleles encode slightly different versions of a gene, which may cause different phenotypic traits. Genes evolve due to natural selection and genetic drift of the alleles. When a gene is duplicated, the new copy can evolve a new function.