Allele

a variation, or form of a gene found at specific genetic locus.  Genetic variation comes from the presence of different combinations of alleles in different individuals.  Organisms contain one allele for each of their sets of chromosome*s.  Diploid organisms, such as humans, have two sets of chromosomes and therefore, two alleles for each gene.  An organism's genotype* is a description of the alleles present at specific genetic loci.