In Mendelian genetics, a dominant gene is an allele that is expressed in the phenotype of an organism, even if the organism carries only one copy of that allele. In this case, the gene for tallness is dominant over the gene for shortness. This means that even though the offspring of the cross between a true-breeding short plant and a tall plant all carry one copy of the gene for shortness, they all express the tall phenotype because the gene for tallness is dominant.
