Ratio of complementary genes is : -

Complementary genes are two non-allelic dominant genes that produce a character only when both are present together (e.g. purple flower colour in sweet pea, studied by Bateson and Punnett).
In the $F_2$ of a dihybrid cross: 9 $A\_B\_$ show the character, while 3 $A\_bb$ + 3 $aaB\_$ + 1 aabb (= 7) do not.
So the ratio is 9 : 7.
(9 : 3 : 4 is recessive epistasis and 12 : 3 : 1 is dominant epistasis.)