genes determine traits, and different versions of genes, called alleles, can lead to variations in the traits that are expressed in the phenotype. Some alleles are dominant and will be expressed in the phenotype regardless of whether they are paired with another dominant or recessive allele, while other alleles are recessive and will only be expressed in the phenotype when paired with another recessive allele. The combination of alleles that an organism possesses for a particular trait is its genotype, and the physical expression of those alleles is its phenotype.