The difference between dominant and recessive traits in genetics is based on allele expression. A dominant trait is expressed phenotypically even when only one dominant allele is present, while a recessive trait requires two copies of the allele for expression. For example, if a dominant allele for brown eyes is present alongside a recessive allele for blue eyes, the individual will have brown eyes. This distinction plays a crucial role in inheritance patterns and helps explain the variability of traits in populations, as described by Mendelian genetics.