What is the difference between a dominant and a recessive allele?

 A dominant allele is an allele that is expressed when present in either a heterozygous or homozygous genotype, while a recessive allele is an allele that is only expressed when present in a homozygous genotype. For example, if an individual has one allele for brown eyes and one allele for blue eyes, the allele for brown eyes is dominant and will be expressed, while the allele for blue eyes is recessive and will not be expressed.