Genes occupy specific positions or loci on chromosomes. Each chromosome contains many genes located at different loci. Genes may have different alleles that reside at a particular locus. The ordered list of loci for a genome is called a gene map, which is used to determine the locus responsible for a phenotype through gene mapping. Chromosomes are divided into bands that are numbered from the centromere to the telomeres, and the chromosomal locus of a gene specifies its location through this banding pattern nomenclature. Different staining techniques can be used to visualize the light and dark bands that describe gene locations on chromosomes.