Here's why:

* Dominant traits are expressed even if only one copy of the dominant allele is present (heterozygous).

* Recessive traits require two copies of the recessive allele (homozygous) to be expressed.

Example:

* Brown eyes are dominant over blue eyes. This means someone with one brown eye allele and one blue eye allele (heterozygous) will have brown eyes.

* To have blue eyes, you need two copies of the blue eye allele (homozygous).