A locus is the place on a chromosome where a specific gene is located, a kind of address for the gene. The plural is "loci," not "locuses."
In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. The ordered list of loci known for a particular genome is referred to as a genetic map. "Mapping" is the process to determine the locus for a particular trait.
A locus can be occupied by any of the alleles of the gene. Diploid or polyploid cells are either homozygous (have the same allele at a locus on all chromosomes) or heterozygous (have different alleles at a locus).
The chromosomal locus of a gene might be given as "9q34.1". Here the 9 is the chromosome number, and the letter q indicates that the position is on its long arm. The letter p is used for the short (petit in French) arm of the chromosome. The numbers following the letter give the position on the arm. When suitably stained, chromosomes have a banded appearance when viewed under a microscope, and each of the bands is numbered, beginning with 1 for the band nearest the centromere. Sub-bands and sub-sub-bands are visible at higher resolution. In the example, the location is on band 3, sub-band 4, sub-sub-band 1.
A range of locations is specified in a similar manner. For example, the locus of gene OCA1 might be given as 11q14-q21, meaning it is on the long arm of chromosome 11, somewhere in the range of sub-band 4 of band 1, and sub-band 1 of band 2.
The ends of a chromosome are labeled ptel and qtel and so "2qtel" refers to the telomere of the long arm of chromosome 2.