What distinguishes a dominant allele in genetics?

A dominant allele is characterized by its ability to be expressed when only one copy is present in the genotype. This means that if an individual possesses at least one dominant allele for a particular gene, that trait will manifest in the phenotype, overshadowing any effects from a recessive allele that may also be present. This fundamental principle plays a key role in patterns of inheritance as defined by Mendelian genetics.

The other options do not accurately define a dominant allele. For example, a dominant allele does not require a second dominant allele for expression, which would be the case for a homozygous dominant trait. The claim that a dominant allele cannot be inherited is incorrect since alleles are passed down from parents to offspring. Finally, a dominant allele does not give rise to recessive traits; it is the recessive alleles that express traits only when two copies are present. This distinction is crucial for understanding how genetic information is transmitted and expressed in living organisms.