Which of the following statements is true about alleles?

Alleles refer to the different forms of a gene that may exist at a specific locus on a chromosome. They can be categorized as either dominant or recessive, which determines how traits are expressed in an organism. A dominant allele can mask the expression of a recessive allele when both are present in an organism's genotype, meaning that the dominant trait will be visible in the phenotype. This dual nature of alleles allows for variation within a population and is fundamental to the principles of inheritance. The existence of both dominant and recessive alleles contributes to genetic diversity and the variation seen in traits among individuals.

This understanding illustrates why the selection that states alleles can be either dominant or recessive is accurate, as it encompasses the full range of genetic expression and inheritance mechanisms seen in biological systems.