* Alleles: These are alternative forms of a gene. For example, the gene for eye color might have a brown allele (B) and a blue allele (b).

* Dominant Allele: This allele masks the expression of the recessive allele when they are paired together. Let's say brown eyes are dominant.

* Recessive Allele: This allele is only expressed if two copies of it are present. In our eye color example, a person needs two "b" alleles to have blue eyes.

Heterozygous: An individual is heterozygous for a trait when they have two different alleles for that gene. In our example, a person with one "B" allele and one "b" allele would have brown eyes because the dominant brown allele masks the recessive blue allele.

Key points:

* A heterozygous individual will always express the dominant trait, even though they carry the recessive allele.

* The recessive trait will only be expressed if the individual is homozygous recessive (has two copies of the recessive allele).

Let me know if you'd like more examples or explanations!