What is the definition of a dominant allele?

A dominant allele is defined as an allele that is always expressed if present in an organism's genotype. This means that even if an individual has just one copy of the dominant allele, its trait will manifest in the phenotype. For example, if brown eyes are a trait governed by a dominant allele, a person with one brown-eyed allele and one blue-eyed allele will display the brown eye phenotype.

The key aspect of a dominant allele is its ability to overshadow the effects of a recessive allele, which requires two copies (homozygous condition) for its trait to be expressed. Since dominant alleles determine the phenotype even when only one copy is present, they play a crucial role in inheritance patterns and genetic expressions in organisms.

In summary, the definition aligns with the nature of a dominant allele in genetics, where it is definitive in influencing the phenotype regardless of whether it is presented alone or paired with a recessive form.