What is the definition of a dominant allele?

The definition of a dominant allele is accurately captured by stating that it expresses its phenotype even in the presence of a recessive allele. This means that when an individual carries one dominant allele and one recessive allele for a particular trait, the dominant allele will determine the phenotype, effectively “overriding” the effect of the recessive allele.

In Mendelian genetics, the presence of just one copy of a dominant allele—regardless of the other allele's nature (dominant or recessive)—is sufficient for the trait associated with that allele to be displayed. This characteristic of dominance is crucial for understanding inheritance patterns and predicting the phenotypic outcomes of genetic crosses.

In contrast, other aspects that may lead to confusion include terms that refer specifically to recessive alleles or situations where alleles need to be homozygous, as these pertain to different genetic concepts. For instance, alleles that are expressed only when homozygous correspond to recessive alleles, which block the expression of dominant counterparts in a heterozygous state. Understanding these distinctions allows for a clearer grasp of how traits are inherited and expressed in generations.