allele: any alternative form of a particular gene. Different alleles often confer different phenotypes

gene: a section of a DNA molecule (or RNA molecule, in some viruses) that contributes to a particular function or phenotype

gene product: the ribonucleic acid (RNA) or protein that is the product of the gene being expressed (transcribed or translated). The RNA or protein often has a name that is the same as, or similar to, the name of the gene, so it is important to distinguish between these entities

locus: the mapped location of a DNA sequence; it is not a synonym for gene. Loci is the plural of locus. See also Chromosomal location