Fundamental niche is the entire set of conditions under which an animal (population, species) can survive and reproduce itself. Realized niche is the set of conditions actually used by given animal (pop, species), after interactions with other species (predation and especially competition) have been taken into account
If you drew a Punnett Square, you will see that the possible outcomes would be AA, Aa, aA, and aa. Three out of the four would exhibit the dominant trait.
