Locus

A locus (plural loci) in genetics is a fixed position on a chromosome, like the position of a gene or a marker (genetic marker).

A locus is a position on a chromosome where a gene or other genetic marker is located, while an allele is a variant of a gene (i.e. a variant form of the DNA sequence located at a specific locus, coding for a specific functional product, associated with a trait)

Each chromosome carries many genes; human's 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 heterozygous with respect to that gene.