A gene is a unit of hereditary information, usually consisting of a sequence of DNA, that encodes a functional product. That product may be a protein or an RNA molecule that functions without being translated into protein. Genes influence biological traits through their products and interactions with other genes and the environment. They are central to genetics, but a gene is not necessarily a self-contained instruction for a single characteristic: its activity and effects depend on biological context. (genome.gov)
Development of the concept
The concept originated in studies of inheritance before its molecular basis was understood. Gregor Mendel presented his pea-plant experiments in 1865, showing that certain inherited characteristics behaved as though transmitted through discrete factors rather than blended between generations. In 1909, Danish botanist Wilhelm Johannsen introduced the word gene for these units of heredity. He also distinguished genotype, an individual's genetic constitution, from phenotype, its observable characteristics. (genome.gov)
The molecular concept connects hereditary units to sequences that produce RNA and proteins. Nevertheless, the precise boundaries of a gene are not always straightforward. A definition may emphasize an inherited unit, a transcribed sequence, or the sequences required to produce a functional product. Associated regulatory regions can affect several genes, making them difficult to assign exclusively to one. (genome.gov)
Organization and structure
Genes occupy positions within a genome, the complete genetic material of an organism. In cellular organisms, they are carried on chromosomes and consist of ordered nucleotide sequences. In eukaryotes, most genetic material lies within the cell nucleus, although mitochondria also contain DNA and genes. The human genome contains approximately 20,000 protein-coding genes; protein-coding sequence accounts for only a small fraction of its total DNA. (genome.gov)
Many eukaryotic genes contain exons and introns. Exons contribute sequence to the mature RNA, whereas introns are removed during RNA splicing. Exons are not necessarily entirely protein-coding: they may include untranslated regions. Through alternative splicing, different combinations of sequence from one gene can enter mature transcripts, allowing a single gene to produce multiple RNA and protein forms. Thus, the number of proteins an organism can produce is not simply equal to its number of genes. (genome.gov)
Expression and regulation
During transcription, an RNA molecule is synthesized using DNA as a template. For a protein-coding gene, the resulting transcript is processed into messenger RNA, which carries sequence information to the protein-synthesis machinery. During translation, a ribosome reads that information according to the genetic code, assembling amino acids into a polypeptide. Other genes produce functional RNAs, including RNAs involved directly in translation or in regulating gene activity. (ncbi.nlm.nih.gov)
Gene expression is regulated rather than occurring uniformly. Different cell types can use different subsets of the same genome, and expression can change during development or in response to environmental conditions. Transcription factors interact with regulatory DNA to influence transcription. Regulation also occurs through RNA processing, RNA stability, translation, and protein turnover. Chemical modifications such as DNA methylation provide additional mechanisms for controlling activity without changing the underlying nucleotide sequence. (ncbi.nlm.nih.gov)
Inheritance and variation
Alternative sequence versions at a genomic location are called alleles. For most human autosomal genes, an individual normally inherits one copy from each parent. Identical alleles at a location constitute a homozygous genotype; different alleles constitute a heterozygous genotype. During meiosis, chromosome copies separate into reproductive cells, while chromosomal crossing-over can exchange segments between corresponding chromosomes and generate new combinations of alleles. (genome.gov)
Dominance and recessiveness describe relationships between alleles and a specified phenotype, not whether a gene is intrinsically stronger or more important. Some traits show relatively simple inheritance patterns; others involve many genes and environmental influences. Consequently, possessing a particular allele does not always determine an observable outcome. Human mitochondrial genes follow a different pattern and are generally inherited through the mother. (medlineplus.gov)
A mutation changes genetic sequence. Changes include substitutions, insertions, deletions, and expansions of repeated sequences. Their effects depend on their location and consequences: they may alter protein structure, interrupt RNA processing, change gene activity, or have no detectable effect. In human genetics, variant is often preferred as a neutral term because a sequence difference does not necessarily cause disease. Germline variants can pass to offspring, whereas variants confined to somatic cells generally cannot. (medlineplus.gov)
Genes in evolution and research
Heritable genetic variation supplies material for evolution. Natural selection can change the frequency of variants when their effects influence reproductive success. Frequencies can also change through genetic drift, the random sampling of variants between generations. A variant's consequences therefore depend both on its biological effects and on population processes; not every common variant is beneficial, and not every rare variant is harmful. (medline-plus.org)
Researchers study genes by determining DNA sequences, locating genes within genomes, measuring RNA production, and examining the effects of sequence changes. Sequence data identify what genetic information is present, while expression measurements reveal which information is being used under particular conditions. Functional interpretation requires connecting these observations to biological mechanisms: a sequence difference alone does not establish what a gene does or whether that difference causes a trait. (genome.gov)