Which of the following describes alleles?

Alleles are defined as different forms of a gene that occupy the same locus on homologous chromosomes. This means that within a population, an allele can represent various alternative sequences of nucleotides, which lead to different traits or expressions of those traits. For example, a gene determining flower color may have several alleles, such as one for red flowers and another for white flowers. Each of these variations produces distinct physical characteristics in the organism.

In contrast, identical forms of a gene would not represent the concept of alleles, as alleles by definition require variability. Chromosomal pairs refer to the structures that hold genes and their alleles but do not pertain directly to the definition of alleles themselves. Lastly, proteins coded by genes are the result of gene expression and do not define what alleles are; rather, they are the products created by genes. Understanding the distinction between these concepts is important in genetics, especially in studying inheritance patterns and variations within species.