Definition
Two independent pairs of genes which interact to produce a trait together, but each dominant gene alone doesn't show its effect, are called complementary Genes.
Example
Several varieties of white flowered sweet peas are known and a cross between most of these varieties yields only white flowered offspring
A cross illustrating the action of Complementary gene
One dominant gene C , controls the synthesis of the raw material for the purple pigment
The Other Dominant gene E, controls the formation of an enzymes that converts the raw material into the purple pigment.
At lease one C gene and one E gene must be present to produce purple flower.
The absence of any one or both results in the formation of white flowers.
So, the cross between ccEE and CCee gives CcEe in F1 generation .
In F2 generation , the cross gives a ratio of 9 Purple and 7 White Flowered plants .
So, F2 generation is
9 Purple flowered plants : 7 White Flowered plants