Genetic architecture refers to the underlying genetic basis of a phenotypic

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 trait. A synonymous term is the 'genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

-phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 map', the way that genotypes map to the phenotypes.

The genotype-phenotype map has been analyzed in terms of several principal axes: epistasis, polygeny, pleiotropy, quasi-continuity, modularity, phenotypic plasticity, robustness, and evolvability.

• Epistasis
  Epistasis
  In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
  
  : when the alleles at one locus
  Locus (genetics)
  In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
  
   change the phenotypic effects of genetic variation at another locus, the two genes are said to exhibit 'epistasis
  Epistasis
  In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
  
  ' in their interactions.
• Polygeny: when multiple genes contribute to a particular phenotypic character, the map is said to possess 'polygeny'. The genetic architecture in cases of polygeny can be further characterized by the spectrum of contributions of the genes, e.g. many genes of small effect vs. few genes of large effect.
• Pleiotropy
  Pleiotropy
  Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
  
  : when multiple phenotypic characters are affected by a single genetic variation, the map is said to possess 'pleiotropy
  Pleiotropy
  Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
  
  '.
• Quasi-continuity: when small genetic changes map to small phenotypic changes, the map is said to possess "quasi-continuity" (Lewontin).
• Modularity
  Modularity
  Modularity is a general systems concept, typically defined as a continuum describing the degree to which a system’s components may be separated and recombined. It refers to both the tightness of coupling between components, and the degree to which the “rules” of the system architecture enable the...
  
  : when two different phenotypic characters can be mapped to mostly non-overlapping sets of genes, the map is said to possess 'modularity
  Modularity
  Modularity is a general systems concept, typically defined as a continuum describing the degree to which a system’s components may be separated and recombined. It refers to both the tightness of coupling between components, and the degree to which the “rules” of the system architecture enable the...
  
  ', though this concept is still in flux in the scientific literature.
• Plasticity
  Phenotypic plasticity
  Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
  
  : when a single genotype gives rise to a spectrum of phenotypes, the phenotype is said to have 'plasticity'. The plasticity may occur as different phenotypes among different individuals of the same genotype, or different phenotypes within the lifetime of a single individual, or different phenotypes in response to specific environmental conditions.
• Mutational robustness
  Mutational robustness
  Mutational robustness describes the extent to which an organism’s phenotype remains constant in spite of mutation. Natural selection can directly induce the evolution of mutational robustness only when mutation rates are high and population sizes are large...
  
  : when the same phenotype occurs in an organism despite a variety of environmental perturbations, it is said to be 'robust'. When the same phenotype is produced despite mutations in the genes involved it its production, it is said to possess 'mutational robustness
  Mutational robustness
  Mutational robustness describes the extent to which an organism’s phenotype remains constant in spite of mutation. Natural selection can directly induce the evolution of mutational robustness only when mutation rates are high and population sizes are large...
  
  '.
• Evolvability
  Evolvability
  Evolvability is defined as the capacity of a system for adaptive evolution. Evolvability is the ability of a population of organisms to not merely generate genetic diversity, but to generate adaptive genetic diversity, and thereby evolve through natural selection.In order for a biological organism...
  
  : when there is a significant chance that genetic variation can be produced which produces a net increase in adaptation in an organism, the genotype-phenotype map is said to have 'evolvability
  Evolvability
  Evolvability is defined as the capacity of a system for adaptive evolution. Evolvability is the ability of a population of organisms to not merely generate genetic diversity, but to generate adaptive genetic diversity, and thereby evolve through natural selection.In order for a biological organism...
  
  '.
• Transgressive phenotype: when a phenotype is more extreme than the phenotype displayed by either of the parental lines.