Allele — One of the different DNA sequence forms that can occur at the same genomic location. In a basic diploid autosomal example, an individual receives one allele from each parent and therefore carries two at that locus.
Understanding by example
If two forms are represented as A and a, an individual may be AA, Aa or aa. These are teaching symbols, not actual gene names or specific morphs. A population can also contain more than two alleles at one locus.
Common misconceptions
One allele does not always correspond to one visible feature. Multiple genes and the environment can influence phenotype. In morph claims, distinguish evidence measured at the DNA level from inferences based on appearance and breeding records.
Related reading and sources
For species-specific context, see Distinguishing Inheritance Patterns in Morphs. Browse the full glossary.
Basic reference: NHGRI: Allele.