Alleles are the different forms of a gene. Alleles are corresponding pairs of genes located at specific positions in the chromosomes.Together, alleles determine the genotyp … e. Alleles which determine some aspect of the phenotype, the physical appearance of an organism, are said to be coding alleles.lleles are the different forms of a gene. Alleles are corresponding pairs of genes located at specific positions in the chromosomes. Together, alleles determine the genotyp … e.