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Phenotype is the set or type to which an organism belongs based on observable anatomical or structural, physiological, biochemical, and behavioral inherited characteristics. These characteristics can be identified by observation of development and morphology (physical structure). Characteristics need not be visible but must be observable in some manner. Characteristics may be discreet, as blood type, or continuously varying, as hair color.
Origin of term
To distinguish it from genotype (genetic set or type), the term phenotype was additionally created by Wilhelm Johannsen in 1911. These terms were similar to August Weismann's concepts of heredity and body. The concepts are based on Gregor Mendel's hypothesis of internal genetic elements that produce external appearances and yet maintain their discrete individuality.
Description of process
- See main article: Gene expression
The formation of an individual organism's phenotype occurs by the information from a gene's DNA being transcribed to an RNA molecule. The information now carried on the RNA molecule codes for the chemical structure of one of various proteins from which cells are built. Thus the phenotype is the result of interpreting instructions derived from the genotype. Note that the genotype is never affected by any aspect of the phenotype. In other words there can be no inheritance of acquired characteristics.
Relationship to genotype
The relationship between genotype and phenotype is many-to-many. That is, there can be different genotypes corresponding to a given phenotype. This can be the result of inactive, recessive, inhibited, or weak genes. The ability to produce the same phenotype despite variability of genotype, or environment, is known as "canalization" or "developmental buffering." Thus one cannot always make inferences about the genotype based on the phenotype.
Similarly a given genotype can correspond to many different phenotypes, making predictions of phenotype from genotype difficult. This can be due to "chained gene effects," biological processes, aging, or random variation. The environment can have such a strong effect on development that phenotype is at times defined as the interaction of a genotype with its environment. "Norm of reaction" refers to the phenotypes expressed for a given genotype in various environments.
The relationship between genotype and phenotype has important implications for evolution. Since phenotypic variation is the raw material for evolution by natural selection, success of a genotype depends on fitness of the expressed phenotype to its environment.
- "The Genotype/Phenotype Distinction". Stanford Encyclopedia of Philosophy. Jan 23, 2004.