Oligogenic inheritance

Oligogenic inheritance (Greek ὀλίγος – "ὀligos" = few, a little) describes a trait that is influenced by a few genes. Oligogenic inheritance represents an intermediate between monogenic inheritance in which a trait is determined by a single causative gene, and polygenic inheritance, in which a trait is influenced by many genes and often environmental factors.
Historically, many traits were thought to be governed by a single causative gene (in what is deemed monogenic inheritance), however work in genetics revealed that these traits are comparatively rare, and in most cases so-called monogenic traits are predominantly influenced by one gene, but can be mediated by other genes of small effect.
History.
Around the 1930s/40s, evidence that multiple genes could affect the risk of disease that showed discrete inheritance patterns, due to differences in the age of onset of disease for siblings. The age of onset for sibling pairs was very similar, but between pairs of siblings could be quite different, and would in some cases cluster into several age brackets. This suggested a major gene that controlled the risk for a disease, and other genes that impacted age of onset.
The recognition of diseases which were influenced by more than one gene highlighted a need to develop methodologies for detecting these oligogenic inheritance patterns, as they did not fit the more straightforward Mendelian model of inheritance. The developments of such methods accelerated the discovery of other examples of oligogenic traits, and sparked a change in the way genetic disease was perceived.
Modifier genes.
One example of oligogenic inheritance is a case where one gene is sufficient to cause a trait, however its penetrance or expressivity is influenced by another gene, called a modifier. An example of such a case is the gene "TGFB1" which modified a person's risk of developing Alzheimer's disease if they are carrying the disease variant of the gene "APP". The mechanism is thought to work through the modifier variant increasing the clearance of amyloid fibers in the aging brain, reducing plaque burden.
Identification of oligogenic traits.
A trait can be recognised as oligogenic through the following lines of evidence: