Which of the following best describes a dominant allele?

A dominant allele is defined by its ability to mask the phenotypic expression of a recessive allele when both are present in a heterozygous organism. This means that when an individual has one dominant allele and one recessive allele for a particular trait, the dominant trait will be expressed in the phenotype, effectively overshadowing the recessive trait. This characteristic is fundamental in understanding Mendelian genetics, where dominant and recessive traits exhibit predictable inheritance patterns.

The other options describe attributes that do not align with the definition of dominant alleles. For instance, the idea that a dominant allele is expressed only in the homozygous state misrepresents how dominance functions; dominant alleles can be expressed in both homozygous (two identical dominant alleles) and heterozygous (one dominant and one recessive allele) states. Similarly, while dominant alleles can indeed lead to disorders, stating that they cause issues in both heterozygous and homozygous states does not capture the uniqueness of their expression relative to recessive alleles. The option stating that a dominant allele is never expressed phenotypically contradicts the core principle that dominant alleles are manifested in the observable characteristics of an organism. Thus, the focus on the ability of