From "Gene Interactions: Allelic and Non-Allelic" (biologydiscussion.com)
'Non-allelic or inter-allelic interactions … occur where the development of single character is due to two or more genes affecting the expression of each other in various ways.'
'Two non-allelic gene pairs affect the same character. The dominant allele of each of the two factors produces separate phenotypes when they are alone. When both the dominant alleles are present together, they produce a distinct new phenotype. The absence of both the dominant alleles gives rise to yet another phenotype.'
'When a gene or gene pair masks or prevents the expression of other non-allelic gene, called epistasis. The gene which produces the effect called epistatic gene and the gene whose expression is suppressed called hypostatic gene.'
From "Genetic Terminology" (ncbi.nlm.nih.gov)
'Whereas Drosophila geneticists used to talk of two loci for a gene, and human geneticists used to talk of two genes at a locus, modern geneticists talk of “two alleles of a gene” or “two alleles at a locus”; this last, which is nowadays so common, is the terminology that will thus be used in this book. It then follows (rather awkwardly) that two alleles at the same locus are allelic to each other, whereas two alleles that are at different loci are non-allelic to each other.'
From slides "Interaction of genes" (slide_9) (slideshare.net)
'Non-Allelic gene interaction: In inter allelic genetic interactions, the independent (non homologous) genes located on the same or on different chromosomes interact with one another for the expression of single phenotypic trait of an organism.'