The term Genotype, will be
explained together with the term Phenotype.
Genotype
The genotype is the genetic makeup of a cell, an organism, or an
individual.
For instance, the human CFTR gene, which encodes a protein that
transports chloride ions across cell membranes, can be dominant
(A) as the normal version of the gene, or recessive (a) as a
mutated version of the gene. Individuals receiving two recessive
alleles will be diagnosed with Cystic fibrosis.
It is generally accepted that inherited genotype, transmitted
epigenetic factors, and non-hereditary environmental variation
contribute to the phenotype of an individual.
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.
Phenotype
A phenotype is the composite of an organism's observable
characteristics or traits, such as its morphology, development,
biochemical or physiological properties, phenology, behaviour,
and products of behaviour (such as a bird's nest). 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.
Phenotypic variation
Phenotypic variation (due to underlying heritable genetic
variation) is a fundamental prerequisite for evolution by natural
selection. It is the living organism as a whole that contributes
(or not) 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 (G) + environment (E) + genotype & environment
interactions (GE) → phenotype (P)
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.
Historical frame
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 (heredity) and somatic cells (the body). 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.
The genotype–phenotype
distinction is drawn in genetics. "Genotype" is an organism's
full hereditary information. "Phenotype" is an organism's actual
observed properties, such as morphology, development, or
behavior. This distinction is fundamental in the study of
inheritance of traits and their evolution.
It is the organism's physical properties which 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 genes
contribute to a trait, and the phenotype is the observable
expression of the genes (and therefore the genotype that affects
the trait). Say a white mouse had both recessive genes that
cause the colour of the mouse to be inactive (so "cc"). Its
genotype would be responsible for its phenotype (the white
colour).
The mapping of a set of genotypes to a set of phenotypes is
sometimes referred to as the genotype–phenotype map.
Similar genotypic changes may result in similar phenotypic
alterations, even across a wide range of species, for example:
a DNA error in a gene necessary for the development of an
eye, would result in a malformed eye in most species.
Identical twins
An organism's genotype is a major 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.
Linguistic derivation
The term Genotype is derived from the
ancient Greek word genes (γένος) meaning "born" or
"race", and týpos (τύπος), meaning "type".
The term Phenotype is derived from the
ancient Greek word phainein / phainō
(φαίνω) meaning "to show, to bring to light, make to
appear", and typos, meaning "type".
External sources
http://en.wikipedia.org/wiki/Genotype
http://en.wikipedia.org/wiki/Phenotype
http://en.wikipedia.org/wiki/Genotype-phenotype_distinction