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The genotype-phenotype distinction must be drawn when trying to understand the inheritance of traits and their evolution. The genotype of an organism represents its exact genetic makeup, that is, the particular set of genes it possesses. Two organisms whose genes differ at even one locus (position in their genome) are said to have different genotypes. The transmission of genes from parents to offspring is under the control of precise molecular mechanisms. The discovery of these mechanisms and their manifestations began with Mendel and comprises the field of genetics.

The term "genotype" refers to the full hereditary information of an organism. The phenotype of an organism represents its actual physical properties, such as height, weight, hair color, and so on. It is the organism's physical properties that directly determine its chances of survival and reproductive output, while the inheritance of physical properties occurs only as a secondary consequence of the inheritance of genes. Therefore, to properly understand the theory of evolution via natural selection, one must understand the genotype-phenotype distinction.

The mapping of a set of genotypes to a set of phenotypes is sometimes referred to as the genotype-phenotype map.

An organism's genotype is a major (the largest by far for morphology) influencing factor in the development of its phenotype, but it is not the only one. Even two organisms with identical genotypes normally differ in their phenotypes. One experiences this in everyday life with monozygous (i.e. identical) twins. Identical twins share the same genotype, since their genomes are identical; but they never have the same phenotype, although their phenotypes may be very similar. This is apparent in the fact that their mothers and close friends can always tell them apart, even though others might not be able to see the subtle differences. Further, identical twins can be distinguished by their fingerprints, which are never completely identical.

The concept of phenotypic plasticity describes the degree to which an organism's phenotype is determined by its genotype. A high level of plasticity means that environmental factors have a strong influence on the particular phenotype that develops. If there is little plasticity, the phenotype of an organism can be reliably predicted from knowledge of the genotype, regardless of environmental peculiarities during development. An example of high plasticity can be observed in larval newts¹: when these larvae sense the presence of predators such as dragonflies, they develop larger heads and tails relative to their body size and display darker pigmentation. Larvae with these traits have a higher chance of survival when exposed to the predators, but grow more slowly than other phenotypes.

In contrast to phenotypic plasticity, the concept of genetic canalization addresses the extent to which an organism's phenotype allows conclusions about its genotype. A phenotype is said to be canalized if mutations (changes in the genome) do not noticeably affect the physical properties of the organism. This means that a canalized phenotype may form from a large variety of different genotypes, in which case it is not possible to exactly predict the genotype from knowledge of the phenotype (i.e. the genotype-phenotype map is not invertible). If canalization is not present, small changes in the genome have an immediate effect on the phenotype that develops.

The terms "genotype" and "phenotype" were created by Wilhelm Johannsen in 1911.

• list of distinctions

1. J. Van Buskirk and B. R. Schmidt, "Predator-induced Phenotypic Plasticity in Larval Newts: Trade-offs, Selection, and Variation in Nature," Ecology 81 (2000): 3009-3028.

• Stanford Encyclopedia of Philosophy entry

An earlier version of this article was posted on Nupedia.