Phenotype

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
Wilhelm Johannsen creat coined the term "phenotype" in 1911 in contradistinction to "genotype" (genetic set or type). These terms had similar functions to August Weismann's concepts of "body" (soma) and "heredity". The concepts stem from 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.

The phenotype of a heterozygote genome can be intermediate between those of the homozygous genomes, or both alleles can be expressed. A gene can code for more than one trait.

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.