Gene-Gene interaction:Gene-gene interaction is a phenomenon where two or more non-allelic genes interact to influence a single outcome or trait. It is important to note that these interactions do not necessarily involve direct interaction between the genes themselves. Instead, the products of different genes contribute collectively to the development of a common phenotype. 

A better example of gene interaction is where an allele of one gene interacts with an allele of another gene by masking its effect. This phenomenon is known as epistasis. 

Epistasis is a genetic phenomenon where the effect of one gene (called an epistatic gene) masks or modifies the effect of another gene (called a hypostatic gene). Essentially, epistasis occurs when the expression of one gene depends on the presence or absence of alleles from another gene. This interaction can alter the expected phenotypic ratios in offspring, making it an important concept in understanding the complexity of genetic traits.