Here's how it works:

1. Letters: Each letter represents a specific gene. Capital letters represent dominant alleles (expressed even if only one copy is present), and lowercase letters represent recessive alleles (only expressed if two copies are present).

2. Squares: The Punnett square is a grid with four squares.

3. Parents: You write the possible alleles of one parent across the top of the square and the possible alleles of the other parent down the side.

4. Combinations: You fill in each square by combining the alleles from the corresponding row and column. This shows all the possible combinations of alleles that their offspring could inherit.

Example:

Let's say we're looking at eye color, with brown eyes being dominant (B) and blue eyes being recessive (b).

* Parent 1: Heterozygous for brown eyes (Bb)

* Parent 2: Homozygous for blue eyes (bb)

| | B | b |

|-----|-----|-----|

| b | Bb | bb |

| b | Bb | bb |

Results:

* Bb: Brown eyes (dominant allele present)

* bb: Blue eyes (two recessive alleles)

The Punnett square shows that there's a 50% chance of their offspring having brown eyes and a 50% chance of having blue eyes.