The principal of Dominance states that

Principle of Dominance was given by Sir Gregor Johann Mendel.

Explanation:

Principle of Dominance was given by Sir Gregor Johann Mendel as a conclusion of the monohybrid cross between the different characteristic pea plants. It says that in an organism which is heterozygous for a particular character, only the dominant trait is able to express as a phenotype, not the recessive one. In other words, the recessive allele is completely masked by the presence of the dominant allele. This leads to same phenotype of a homozygous dominant organism and a heterozygous organism.

To test the organism about its heterozygosity, the organism is led through a test cross where the organism is crossed with a recessive organism. If the F1 offsprings are all dominant, then the organism is homozygous dominant, and if the F1 offsprings are of half dominant and half recessive phenotype, then the organism is heterozygous.