Here's how allele combinations can produce a trait controlled by a dominant allele:

* Homozygous Dominant: Two copies of the dominant allele (e.g., AA). The trait will be expressed.

* Heterozygous: One copy of the dominant allele and one copy of the recessive allele (e.g., Aa). The dominant allele will mask the recessive allele, and the trait will still be expressed.

Example:

Let's say brown eyes are dominant (B) and blue eyes are recessive (b):

* BB: Homozygous dominant, brown eyes.

* Bb: Heterozygous, brown eyes.

* bb: Homozygous recessive, blue eyes.

Key Points:

* Dominant alleles exert their effect even when only one copy is present.

* Recessive alleles only exert their effect when two copies are present.

Let me know if you'd like more examples or want to explore specific genetic scenarios!