Which of the following best describes a dominant allele?

A dominant allele is characterized by its ability to mask the expression of a recessive allele when both are present in an organism. This means that if at least one dominant allele is present in the genotype, its corresponding trait will be expressed in the organism's phenotype, regardless of the presence of a recessive allele. This fundamental concept in genetics explains why dominant traits appear more frequently in a population compared to recessive traits, which require two copies to be expressed.

Other options do not accurately capture the nature of dominant alleles. For example, stating that a dominant allele must be present in two copies to be expressed incorrectly describes its behavior, as it can be expressed in both homozygous and heterozygous combinations. Additionally, although dominant alleles can be inherited, they are not always passed from one parent to offspring; it depends on the genetic makeup of the parents. Lastly, the idea that a dominant allele is expressed only in homozygous individuals is misleading, as dominant alleles do indeed exert their influence when heterozygous. Understanding these distinctions helps in grasping genetic inheritance patterns accurately.