A gene is a segment of DNA that encodes the information needed to produce a functional product — usually a protein, sometimes a functional RNA molecule. Genes are the units of heredity: they are passed from parent to offspring through reproduction, and the variation among genes within a population is the material on which natural selection acts.
The concept of the gene has changed substantially since Gregor Mendel’s experiments with pea plants in the 1860s. Mendel identified discrete “factors” that determined traits like flower color and seed shape, and he showed that these factors segregated independently during reproduction. The molecular identification of genes with segments of DNA came a century later. But the clean picture of “one gene, one protein, one trait” has not survived closer examination. Most traits are influenced by many genes (polygenic), many genes influence more than one trait (pleiotropy), and gene expression depends on regulatory context, epigenetic modification, and environmental conditions.
Genes come in variant forms called alleles. Different alleles of the same gene may produce different versions of the same protein, or different levels of expression, leading to phenotypic variation within a population. This variation is the substrate of evolution: natural selection favors alleles that increase reproductive success in a given environment, genetic drift changes allele frequencies by chance, and mutation introduces new alleles.
The genotype of an organism is its full complement of genes and alleles. The phenotype — the observable characteristics — is not a direct readout of the genotype but the product of gene expression during development, modulated by environment and stochastic processes. Two organisms with identical genotypes can have different phenotypes if they develop under different conditions.
Related terms
- DNA — the molecule that genes are made of
- Phenotype — the observable outcome of gene expression in context
- Natural selection — the evolutionary process that acts on genetic variation
- Cell — the structure within which genes are expressed