Here's a breakdown:

* Gene: A segment of DNA that codes for a specific trait.

* Allele: A variation of a gene.

* Heterozygous: Having two different alleles for a particular gene.

Example:

Let's say the gene for eye color has two alleles:

* B: Brown eyes

* b: Blue eyes

A person with the genotype Bb is heterozygous for eye color. They have one allele for brown eyes and one allele for blue eyes.

In contrast, a person with the genotype BB is homozygous dominant, and a person with the genotype bb is homozygous recessive.