The Law of dominance states that when a pair of alleles or allelomorphs are brought together in F₁ hybrid, then only one of them expresses itself, masking the expression of the other completely. Monohybrid cross was made to study the simultaneous inheritance of a single pair of Mendelian factors. The cross in which only alternate forms of a single character are taken into consideration is called a monohybrid cross. The trait which appeared in the F₁ generation was called dominant and the other which did not appear in the F₁ population was called recessive.

Thus, when a pair of alleles are brought together in an F₁ hybrid, then only one of them expresses itself masking the expression of the other completely. In the above example, in Tt - F₁ hybrid (tall) only ‘T’ expresses itself so dominant, and ‘t’ is masked so recessively. Thus, this’ proves and explains the law of dominance.