Here's why:

* Recessive alleles only express their trait if two copies of that allele are present.

* Dominant alleles express their trait even if only one copy is present.

Example:

* Let's say a gene for eye color has two alleles: B (brown eyes) and b (blue eyes).

* If someone has the genotype BB or Bb, they will have brown eyes because B is dominant.

* Only if someone has the genotype bb will they have blue eyes because both copies of the allele need to be recessive.

Let me know if you have any other questions!