Additive genetics
Additive genetics are genes that add together to create an effect on a trait. It happens if multiple genes affect the same trait or phenotype. This can be genes for height or genes for if someone gets a disease. For example, one gene might cause someone to be a millimeter taller. A different gene might cause someone to be two millimeters taller. Since the effect of the genes add together, having both of these genes would result in being three (one plus two) millimeters taller.