One of Mendel's Laws of Inheritance. This law states an organism has two different alleles for a trait and the allele that is expressed in the phenotype, masking the expression of the other allele, is said to be dominant. The allele whose expression is masked is said to be recessive.

This is Law of Dominance.

Explanation:

On crossing pure bred pea plants having one trait of a character with pure bred plants having a different trait of the same character, Mendel noticed that all the plants in the first filial generation were having only one trait though they received the factor for other trait. However, when he allowed these plants of first filial generation to pollinate themselves, the other trait also appeared in some plants of second filial generation. And thus the law of dominance was framed, which stated that, characters are controlled by discrete units called factors, these factors occur in pair and in a dissimilar pair of character one factor dominates and the other fails to appear.

Mendelian's Law of Dominance