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Phenotype is the set of observable characteristics of an organism, arising from the interaction between its genetic makeup (genotype) and the environment. Phenotypes encompass not only physical appearance, such as morphology and coloration, but also physiological properties, biochemical pathways, and behaviors. They can be measured at many scales, from cellular phenotypes like enzyme activity to organismal traits like growth rate or reproductive behavior.

The phenotype results from a complex interplay of genetic information, gene regulation, epigenetic modifications, and environmental influences. Gene expression patterns, developmental processes, and stochastic events during growth all contribute to the final traits expressed. Environmental factors such as nutrition, stress, temperature, and social context can cause phenotypic changes without altering the underlying genotype, a phenomenon known as phenotypic plasticity.

Phenotypes are often categorized as morphological, physiological, behavioral, or molecular. Examples include leaf shape in plants, blood glucose levels in animals, mating calls in birds, and protein or metabolite concentrations in cells. Many traits are polygenic, influenced by multiple genes, and they may display incomplete penetrance or variable expressivity, meaning individuals with the same genotype can show different phenotypes.

Measuring phenotypes, or phenotyping, is central to genetics, breeding, ecology, and medicine. Advances in high-throughput phenotyping and phenomics aim to link specific genotypes to detailed phenotypic profiles. Understanding phenotypes supports crop improvement, disease classification, and personalized approaches to treatment by mapping how genetic differences translate into observable traits. The study of phenotypes remains a key bridge between genotype and function in biology.