Here's why:

* Eye color is determined by multiple genes, but one key gene involves two alleles: One allele codes for brown eyes (B), and one allele codes for blue eyes (b).

* A heterozygous individual has one of each allele: They have the combination Bb.

* The brown eye allele (B) is dominant: This means that even with one copy of the blue eye allele (b), the brown eye trait will be expressed.

Therefore, someone with brown eyes could carry the gene for blue eyes even though they don't express it. This is a classic example of how a heterozygous combination of alleles can lead to a specific phenotype (brown eyes).