Epistasis refers to the interdependence of separate genes and their effects. These interactions can be vast and subtle, and create many unexpected outcomes. It also enables a huge diversity of phenotype and potential for evolution. In this silly example, a gene inducing baldness creates an outcome that interjects the outcome of genes for hair colour, as the hair would not get to show itself anyway.
Through this interaction, the different alleles for the colour gene are suddenly and non-linearly disabled.
Continuous variation can also be seen with genetically-induced traits e.g. height, hair colour; where outcomes are found on a spectrum with high diversity, and is also subject to environmental influence e.g. nutrition, light exposure.
Phenotypes can be subject to the action of multiple genetic components. This is polygenic inheritance. For example, heart function is not dependent on a gene. It is a cumulative outcome of many different genes that determine muscle fibres, cell membrane proteins, nerve function, etc.