According to Mendel's Law of Dominance:

1. Characters are controlled by discrete units called factors.
2. Factors occur in pairs.
3. In a dissimilar pair of factors (heterozygous), one member of the pair dominates (dominant) the other (recessive) .

For example, the allele for tallness (T) is dominant over the allele for dwarfness (t), which is why the F1 hybrid (Tt) expresses the tall phenotype .