Epistasis is a genetic phenomenon where the effect of one gene is influenced by one or more other genes. This interaction can mask or modify the expression of traits, leading to variations in physical characteristics. For example, if one gene determines flower color and another gene affects pigment production, the first gene's effect may be hidden by the second. In Mendelian genetics, epistasis can complicate the inheritance patterns predicted by simple dominant and recessive traits. It highlights the complexity of genetic interactions, showing that traits are often the result of multiple genes working together rather than a single gene's influence.