Shows if a gene is assorting independently or autosomally linked. If the genes are assorting independently -> ratio of 1:1:1:1: gene interaction. When two or more genes influence the same phenotypic trait e. g. ) One gene coats for coat colour (b= black b= brown) Other gene coats for the intensity of the colour (d= dense pigment d= dilute) An allele at one gene locus masks the phenotypic expression of the alleles of another gene locus. Can be dominant or recessive: recessive epistasis. When homozygous ee is present, no matter what the other gene (b or bb) is present, the coat colour is always cream: dominant epistasis. Genes on the same chromosome does not assort independently. Cis linkage: when both dominant alleles are on the same chromosome. During meiosis, cross over takes place which leads to formation of ab ab in trans arrangement. Crossing over (chiasma) -> exchange of genetic materials between homologous pairs or sister chromatids.