Genes come in different versions, otherwise known as alleles, with dominant alleles being expressed over recessive alleles (one stronger than the other in terms of traits and etc). Recessive alleles are only expressed when no dominant allele is present. Color of the eyes, for example is allele A is brown color eyes and B is blue color eyes, and assume that A is the dominant then that relationship will prove A to be the simple dominant and B the recessive.
