Dominant alleles are inherited genetic instructions that hide the presence of contrasting instructions. The dominance of these instructions is observable in the physical traits of the organism.
All offspring receive one gene from each parent. These genes contain instructions for a particular trait. These instructions may be the same, or they may be different. Differing genes are called alleles.
Receiving the same instructions from each parent results in the expression of that instruction for the trait. Receiving differing instructions from each parent results in the expression of the instruction from the dominant allele. The allele that is hidden by the dominant allele is called the recessive allele.