When dominant epistasis is operative between two genes loci, the classical 9:3:3:1 ratio becomes modified into
12:3:1
When dominant allele 'A' masks the expression of 'B' 'A' is epistatic gene of 'B'. A can express itself only in the presence of 'B' or b allele. Therefore it is called dominant epistasis. B expresses only when 'aa' is present. Therefore, in 9: 3: 3: 1 ratio both 9 and 3 are expressing the 'A' gene the ratio is now 12: 3: 1.