Genetic locus

A locus (plural loci) in genetics is the position on a chromosome. Each chromosome carries many genes; humans' estimated 'haploid' protein coding genes are 19,000-20,000, on the 23 different chromosomes. A variant of the similar DNA sequence located at a given locus is called an allele. The ordered list of loci known for a particular genome is called a gene map. Gene mapping is the process of determining the locus for a particular biological trait.

Diploid and polyploid cells whose chromosomes have the same allele of a given gene at some locus are called homozygous with respect to that gene, while those that have different alleles of a given gene at a locus, are called the heterozygous with respect to that gene.

The chromosomal locus of a gene might be written 3p22.1, where

Thus, the entire locus is read as "three P two two point one."

The cytogenetic bands are counting from the centromere out toward the telomeres.

A range of loci is specified in a similar way. For 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.

The ends of a chromosome are labeled "pter" and "qter", and so "2qter" refers to the terminus of the long arm of chromosome 2.

A centisome (not to be confused with a centrosome) is defined as 1% of a chromosome length.

Michael, R. Cummings. (2011). Human Heredity. Belmont, California: Brooks/Cole

...
Wikipedia