What is the phenomenon called when one gene pair interacts with a second pair to control its expression by either suppressing or modifying it?

Epistasis is the phenomenon where one pair of gene interacts with a second pair to control its expression by masking, suppressing or modifying it. The term meant that a different gene masks the phenotypic effect of another gene. Moreover, epistasis has a large influence on the evolution of phenotypic traits. 

within 30 of the equator hope this

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.