When both dominant and recessive alleles are present within a single nucleus, the organism is heterozygous for that trait.

Here's why:

* Heterozygous means having two different alleles for a specific gene.

* Dominant allele: The allele that expresses its trait even when paired with a recessive allele.

* Recessive allele: The allele that only expresses its trait when paired with another recessive allele.

For example, if someone has brown eyes (dominant trait) and one parent has blue eyes (recessive trait), they are heterozygous for eye color. They carry both the brown eye allele and the blue eye allele, but their eye color is brown because the dominant allele masks the recessive one.