Here's why:

* Dominant alleles only need to be present on one chromosome to be expressed.

* Homozygous means having two identical alleles for a trait (e.g., AA or aa).

* Heterozygous means having two different alleles for a trait (e.g., Aa).

Therefore, a dominant phenotype can be expressed in both homozygous dominant (AA) and heterozygous (Aa) individuals.

Example:

* Brown eyes are dominant over blue eyes.

* A person with the genotype BB (homozygous dominant) will have brown eyes.

* A person with the genotype Bb (heterozygous) will also have brown eyes, even though they carry one allele for blue eyes.

Only a recessive phenotype will only be expressed when an individual is homozygous recessive (aa).