What decides whether a trait is dominant or recessive? Every chromosome carries genes at specific spots called loci. Each gene has at least two versions called alleles (different forms of the same gene). You draw a chromosome on paper in the typical X shape. Then you mark loci along it and invent fictional genes. Each gene gets at least two alleles. You label each pair as dominant-recessive or co-dominant (where both alleles show equally). The finished drawing shows how allele combinations determine which traits appear. You predict what happens when someone inherits one dominant and one recessive copy.