Example of bands. A locus (plural loci) is the specific location or position of a gene, DNA sequence, on a chromosome, in the field of genetics. Each chromosome carries many genes; humans' estimated ' haploid ' protein coding genes are 20,000-25,000, on the 23 different chromosomes.or example, the locus of gene OCA1 may be written 11q1.4-q2.1, meaning it is on the long arm of chromosome 11, somewhere in the range from sub-band 4 of region 1 to sub-band 1 of region 2.