This condition is known as genetic equilibrium
In genetic equilibrium, there are a number of assumptions that are made in order to obtain the conditions that are described. Note that these assumptions cannot exist in real life and genetic equilibrium is a hypothetical state.
-No gene mutation
-Large population
-No immigration/emigration
-No natural selection
-Random mating
Through these conditions, the occurrence of all phenotypes within an organism's population remains relatively the same.