Final answer:
Epistasis occurs when one gene influences the expression of another gene, modifying the phenotype that is produced. It shows the complexity of genetic interactions beyond the simple Mendelian inheritance of traits being controlled by a single gene. Examples include human eye color and skin color, which involve multiple genes contributing to the final phenotype.
Step-by-step explanation:
When the presence of one gene influences the expression of the phenotype controlled by a second gene, this phenomenon is known as epistasis. This type of genetic interaction can result in an allele of one gene masking or modifying the expression of an allele for a different gene. Gregor Mendel's studies in pea plants originally suggested that each characteristic was controlled by a single gene. However, we now understand that traits are often under the influence of multiple genes. For instance, eye color in humans is not determined by a single gene but is rather an example of polygenic inheritance, with at least eight genes contributing to the phenotype.
Mendelian inheritance, which is the inheritance of traits controlled by a single gene with two alleles, is sometimes oversimplified compared to the complexity of epistatic interactions. For example, the gene for cleft chin is often considered a dominant trait, with the dominant allele producing the cleft chin phenotype whether a person is homozygous (BB) or heterozygous (Bb) for that allele. However, in cases of epistasis, such as with human skin color or height, these traits result from the combined effects of many genes, making the inheritance patterns more complex.