What describes the term 'allele'?

An allele is defined as a variant form of a gene that occurs at a specific locus on a chromosome. Each gene has multiple alleles, which can contribute to different traits or characteristics in an organism. For example, a gene that determines flower color might have several alleles, resulting in flowers that are red, white, or purple. This variation is essential in genetics, as it allows for diversity within a population and is fundamental to the principles of inheritance, as outlined by Mendelian genetics.

The other options represent different concepts in genetics. The specific locations of genes on chromosomes refer to loci, not alleles. The visible expression of a genotype is known as the phenotype, which is separate from the genetic variation represented by alleles. Lastly, the overall genetic structure of an organism refers to its entire genome and not to the specific variations that alleles represent. Understanding these distinctions is crucial in the study of genetics and its application in fields like medicine and agriculture.