A phenotype is the composite of an organism's observable characteristics or traits, such
as its morphology, development, biochemical or physiological properties, phenology, behavior,
and products of behavior.
A phenotype results from the expression of an organism's genes as well as the influence
of environmental factors and the interactions between the two.
When two or more clearly different phenotypes exist in the same population of a species,
the species is called polymorph.
The genotype of an organism is the inherited instructions it carries within its genetic
code.
Not all organisms with the same genotype look or act the same way because appearance and
behavior are modified by environmental and developmental conditions.
Likewise, not all organisms that look alike necessarily have the same genotype.
This genotype-phenotype distinction was proposed by Wilhelm Johannsen in 1911 to make clear
the difference between an organism's heredity and what that heredity produces.
The distinction is similar to that proposed by August Weismann, who distinguished between
germ plasm and somatic cells.
The genotype-phenotype distinction should not be confused with Francis Crick's central
dogma of molecular biology, which is a statement about the directionality of molecular sequential
information flowing from DNA to protein, and not the reverse.
Difficulties in definition Despite its seemingly straightforward definition,
the concept of the phenotype has hidden subtleties.
It may seem that anything dependent on the genotype is a phenotype, including molecules
such as RNA and proteins.
Most molecules and structures coded by the genetic material are not visible in the appearance
of an organism, yet they are observable and are thus part of the phenotype.
Human blood groups are an example.
It may also seem that this goes beyond the original intentions of the concept with its
focus on the organism in itself, meaning that the lowest level of biological organization
compatible with the phenotype concept is at the cellular level.
Either way, the term phenotype includes traits or characteristics that can be made visible
by some technical procedure.
A notable extension to this idea is the presence of "organic molecules" or metabolites that
are generated by organisms from chemical reactions of enzymes, e.g. vitamins, that can be scored
as phenotype.
Another extension adds behavior to the phenotype, since behaviors are also observable characteristics.
Indeed there is research into the clinical relevance of behavioral phenotypes as they
pertain to a range of syndromes.
Often, the term "phenotype" is incorrectly used as a shorthand to indicate phenotypical
changes observed in mutated organisms.
Phenotypic variation Phenotypic variation is a fundamental prerequisite
for evolution by natural selection.
It is the living organism as a whole that contributes to the next generation, so natural
selection affects the genetic structure of a population indirectly via the contribution
of phenotypes.
Without phenotypic variation, there would be no evolution by natural selection.
The interaction between genotype and phenotype has often been conceptualized by the following
relationship: genotype + environment → phenotype
A more nuanced version of the relationship is:
genotype + environment + genotype & environment interactions → phenotype
Genotypes often have much flexibility in the modification and expression of phenotypes;
in many organisms these phenotypes are very different under varying environmental conditions.
The plant Hieracium umbellatum is found growing in two different habitats in Sweden.
One habitat is rocky, sea-side cliffs, where the plants are bushy with broad leaves and
expanded inflorescences; the other is among sand dunes where the plants grow prostrate
with narrow leaves and compact inflorescences.
These habitats alternate along the coast of Sweden and the habitat that the seeds of Hieracium
umbellatum land in, determine the phenotype that grows.
An example of random variation in Drosophila flies is the number of ommatidia, which may
vary between left and right eyes in a single individual as much as they do between different
genotypes overall, or between clones raised in different environments.
The concept of phenotype can be extended to variations below the level of the gene that
affect an organism's fitness.
For example, silent mutations that do not change the corresponding amino acid sequence
of a gene may change the frequency of guanine-cytosine base pairs.
These base pairs have a higher thermal stability than adenine-thymine, a property that might
convey, among organisms living in high-temperature environments, a selective advantage on variants
enriched in GC content.
The Extended Phenotype
The idea of the phenotype has been generalized by Richard Dawkins in The Extended Phenotype
to mean all the effects a gene has on the outside world that may influence its chances
of being replicated.
These can be effects on the organism in which the gene resides, the environment, or other
organisms.
For instance, a beaver dam might be considered a phenotype of beaver genes, the same way
beavers' powerful incisor teeth are phenotype expressions of their genes.
Dawkins also cites the effect of an organism on the behaviour of another organism as an
example of the extended phenotype.
The smallest unit of replicators is the gene.
Replicators cannot be directly selected upon, but they are selected on by their phenotypic
effects.
These effects are packaged together in organisms.
We should think of the replicator as having extended phenotypic effects.
These are all of the ways it affects the world, not just the effects the replicators have
on the body in which they reside.
Phenome and phenomics Although a phenotype is the ensemble of observable
characteristics displayed by an organism, the word phenome is sometimes used to refer
to a collection of traits, while the simultaneous study of such a collection is referred to
as phenomics.
See also Ecotype
Endophenotype The Extended Phenotype
Potentiality and actuality References
External links Mouse Phenome Database
Human Phenotype Ontology Europhenome: Access to raw and annotated mouse
phenotype data "Wilhelm Johannsen's Genotype-Phenotype Distinction"
by E. Peirson at the Embryo Project Encyclopedia