What gene results in a displayed trait only when there are twp paired together?

Genetics: Genetics is how a certain trait gets expressed in a person. Sometimes it is simple, like brown eyes or blue eyes, where if a person has 2 blue genes, the eyes will be blue, but if there is only 1 brown gene, the eyes will be brown because the brown gene is dominant. There are other genes that are more complex, and can express themselves only when there are certain environmental factors present.