It is worth elaborating on the concept of the allele because it can be difficult to understand the distinction between this and the gene. A gene is a functional unit of genetic material that when expressed produces some gene product that may or may not be translated into a protein. Consider a hypothetical gene for eye colour. More specifically, let this gene code for a pigment protein that is secreted into the tissues of the cornea. This protein may have different reflective properties in light owing to structure that can cause some versions to give blue eyes and some brown. There is an effectively direct association between protein structure and the genetic sequence. Thus, different proteins correspond to sequentially distinct genes. We distinguish between these versions of the gene by calling them alleles.
Despite their differences, the blue and brown alleles are both the same gene for eye colour.