Glossary
gene
A gene is a section of DNA that carries the instructions for a trait or a protein, and the alternative versions of a gene are its alleles.
A gene is a section of DNA that carries the instructions for a trait or for a protein, and it sits at a fixed position, its locus, on a chromosome. A human body cell carries two copies of most genes, one on each chromosome of a homologous pair, and the two copies may be identical or may differ. Those alternative versions are alleles, and which ones an organism carries is its genotype. The instructions are read by the central dogma: a gene is transcribed into RNA, and the RNA is translated into a protein, which then does the work that produces the trait. A change in the DNA sequence of a gene is a mutation, and it may change the protein, do nothing, or stop the protein being made at all. The human genome holds roughly 20,000 protein-coding genes, which is far fewer than the number of different proteins a body makes — one gene can yield several products, because the RNA transcript can be spliced in more than one way.
Taught in
Related terms
- allele — An allele is one version of a gene; a person carries two for most genes, one on each chromosome of a homologous pair, and the pair is the genotype.
- chromosome — A chromosome is one long DNA molecule wound around histone proteins, and a human body cell carries 46 of them, in 23 pairs.
- genotype — A genotype is the pair of alleles an organism carries for a gene, written as letters such as BB, Bb or bb.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)