What are the pair of alleles for a trait and how do they determine the expression of that trait?

Alleles are pairs of genes that determine a specific trait in an organism. Each parent contributes one allele, and the combination of alleles determines how the trait is expressed. If the alleles are the same (homozygous), the trait will be expressed in a certain way. If the alleles are different (heterozygous), one allele may be dominant and determine the trait's expression, while the other may be recessive and not expressed.