When the effects of genes add up to make the phenotype, they are called genes?

additive genes.

Explanation:

Additive genes are found in Polygenic Inheritance.

In this pattern of inheritance, the phenotype is conditioned by two or more pairs of allele genes, one of which is called the additive gene, and the other is the indifferent or nonadditive gene. Each additive gene present in an individual determines the increase in phenotype expression intensity, no matter which pair this additive gene is. Nonadditive genes add nothing to phenotype expression.