The situation in which one gene hides the effect of a second gene when both are present in a chromosome is called

The situation in which one gene hides the effect of a second gene when both are present in a chromosome is called Epistasis.
Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes.
The concept of epistasis originated in genetics in 1907 but is now used in biochemistry, computational biology and evolutionary biology.