A phenotype is the set of observable or measurable characteristics of an organism, or a particular characteristic considered individually. It includes physical features, biochemical properties, physiological functions, and behavior—not merely outward appearance. Phenotype arises through the interaction of an organism’s genotype with environmental conditions. Examples include height, pigmentation, blood type, and metabolic activity. In genetics, the distinction between phenotype and genotype separates what an organism exhibits from the genetic information it carries. (genome.gov)
Origins and scope
The Danish botanist Wilhelm Johannsen introduced the phenotype–genotype distinction in 1909, alongside the term gene. This terminology established a conceptual separation between inherited genetic constitution and the characteristics observed in an individual. It remains fundamental to interpreting biological variation: a difference in appearance does not, by itself, establish a difference in genetic constitution. (genome.gov)
“Observable” includes characteristics detected through instruments or laboratory tests. An organism’s phenotype may therefore encompass internal biochemistry, physiology, and responses to experimental conditions. A particular trait is one component of the overall phenotype; researchers often use “phenotype” more narrowly to mean the trait or collection of traits being investigated. (genome.gov)
Phenotypic descriptions can operate at different biological levels. Whole-organism measurements include body size and activity, while a cell can be characterized by its structure, molecular markers, and functional behavior. Phenotype databases consequently include cellular, anatomical, metabolic, and behavioral categories rather than a single measure of appearance. (informatics.jax.org)
Relationship to genotype
A genotype describes genetic information at one or more positions in DNA, or, more broadly, an organism’s genetic constitution. At an individual locus, it may identify a pair of alleles, conventionally represented by symbols such as AA, Aa, and aa. Phenotype describes the resulting characteristic, not the DNA sequence itself. Genotyping and phenotyping are therefore distinct forms of investigation. (genome.gov)
The relationship is usually not one-to-one. Different genotypes can produce similar observable characteristics, and individuals carrying the same relevant genetic variant can exhibit different outcomes. Many characteristics are polygenic traits, influenced by multiple genes as well as environmental factors. Human height, for example, cannot generally be explained by a single locus. (genome.gov)
Gene expression provides a connection between genetic information and phenotype by producing functional RNA or protein products. These products can also regulate other genes. Variation in regulatory activity helps explain why the phenotypic effect of genetic information depends on biological context rather than simply on the presence of a particular sequence. (genome.gov)
Two concepts distinguish aspects of this variability. Penetrance concerns the proportion of individuals with a particular genotype or variant who exhibit an associated phenotype. Expressivity concerns the extent or form of its manifestation. In clinical genetics, reduced penetrance means that some variant carriers do not exhibit the associated condition, whereas variable expressivity means that its features differ among affected individuals. (genome.gov)
Environment and phenotypic plasticity
Environmental influences include nutrition and other exposures experienced during development and adult life. Their effects may modify the expression of genetic differences. A genotype–environment interaction occurs when the effect of genotype changes across environments, or equivalently when different genotypes respond differently to the same environmental change. This is more specific than merely observing that genes and environment both matter. (genome.gov)
Phenotypic plasticity is the capacity of a genotype to produce different phenotypes under different environmental conditions. It can involve morphology, physiology, behavior, or life-history characteristics. Plastic responses can be continuous or can produce distinct alternative forms. Their existence does not automatically establish that they improve survival or reproduction. (pmc.ncbi.nlm.nih.gov)
A reaction norm describes the phenotypes a genotype expresses across a range of environments. Reaction norms distinguish environmental responsiveness within a genotype from differences between genotypes. Their shapes also help characterize genotype–environment interaction: genotypes may differ in the magnitude or direction of their responses. (pmc.ncbi.nlm.nih.gov)
Variation and heritability
Quantitative genetics investigates variation in measurable characteristics, particularly traits influenced by many genetic factors. Heritability expresses how much variation in a trait within a population is attributable to genetic variation under the conditions studied. It concerns differences among individuals, not the proportion of an individual’s phenotype “caused by genes.” (medlineplus.gov)
Heritability estimates are specific to populations and environments. A high estimate does not mean a characteristic is unchangeable, nor does it show that environmental influences are unimportant. Consequently, an estimate obtained in one population cannot automatically explain phenotypic differences between populations living under different conditions. (medlineplus.gov)
Measurement and research uses
Phenotyping is the systematic characterization of phenotypic traits. Methods range from anatomical observation and behavioral testing to biochemical assays and instrument-based measurements. In laboratory mice, for example, researchers measure body composition, glucose tolerance, temperature, food intake, and metabolic activity. Phenotype records commonly include experimental protocols and information about diet, age, and genetic background because these affect interpretation. (jax.org)
Standardized terminology makes results easier to compare and retrieve from a database. The Mammalian Phenotype Ontology organizes descriptions hierarchically, from broad categories to specific abnormalities, and connects annotations with supporting research. Such records distinguish a measured normal phenotype from a characteristic that has not been analyzed—a crucial difference when interpreting missing observations. (informatics.jax.org)