Which type of allele can express its trait with only one copy present?

A dominant allele is the type of allele that can express its trait with only one copy present. This is because a dominant allele's presence in either a homozygous (two identical alleles) or heterozygous (one dominant and one recessive allele) genotype will result in the expression of that trait. For instance, if an organism has one dominant allele for a particular trait, that trait will manifest regardless of whether the other allele is dominant or recessive. This characteristic of dominance is fundamental to Mendelian genetics, where traits can be observed based on the dominance hierarchy established between alleles.

In contrast, a recessive allele requires two copies (homozygous condition) to express its trait, meaning that if there is only one dominant allele present, the trait linked to the recessive allele will not be seen. Codominant alleles require both alleles to be expressed equally, such as in the ABO blood group system wherein both A and B alleles can be expressed simultaneously. Multiple alleles refer to the existence of more than two alleles for a particular gene in a population, but this concept does not directly pertain to the question of expression with only one copy.