Locus (Genetics)

In classical and molecular genetics, a locus (plural loci, from the Latin word for "place") is the fixed chromosomal coordinate occupied by a particular gene, transcriptional regulatory element, or polymorphic DNA sequence marker within a species' genome. Whereas a locus defines the invariant chromosomal address along the linear deoxyribonucleic acid (DNA) molecule of a chromosome, the alternative sequence variants that can reside at that address across different chromosomes in a population are termed alleles. Depending on the analytical framework, a genetic locus can be specified at three levels of resolution: cytogenetically by stained metaphase chromosome band nomenclature (e.g., 7q31.2), genetically by recombination frequency in centimorgans (cM) along a linkage map derived from meiosis, or by nucleotide base-pair coordinates (bp) on a physical reference genome assembly. Understanding the allelic states, linkage relationships, and chromatin architecture of genomic loci forms the foundation of Mendelian disease mapping, population genetics, quantitative heritability analysis, and evolution.

ISCN Banding Coordinates (p and q Arms, Regions, Bands, Sub-bands)

Before whole-genome DNA sequencing became available, human and mammalian geneticists localized genes to visible cytogenetic landmarks on mitotic metaphase chromosomes treated with trypsin and Giemsa stain (G-banding). G-banding produces a reproducible pattern of alternating dark (AT-rich, gene-poor, late-replicating heterochromatic G-bands) and light (GC-rich, gene-dense, early-replicating euchromatic R-bands) transverse stripes along each chromosome. Standardized in 1971 by the Paris Conference and maintained by the International System for Human Cytogenomic Nomenclature (ISCN), every chromosomal locus is designated by an alphanumeric address consisting of four hierarchical components:

1. Chromosome Identifier: The autosome number (1 through 22, ordered roughly from largest to smallest in humans) or sex chromosome letter (X or Y).
2. Chromosomal Arm Symbol: The centromere divides each chromosome into two arms: p (from the French petit, designating the short arm, oriented at the top of idiograms) and q (the next letter in the alphabet, designating the long arm).
3. Region and Band Number: Each arm is divided into cytogenetic regions numbered outward from the centromere toward the telomere (1, 2, 3, ...), followed without punctuation by the specific light or dark band number within that region.
4. Sub-band and Sub-sub-band (Post-Decimal): When higher-resolution prophase or prometaphase chromosome preparations (550 to 850 bands per haploid set) split a metaphase band into finer subdivisions, a decimal point is placed after the band number followed by the sub-band digit(s).

Thus, the human cystic fibrosis transmembrane conductance regulator (CFTR) locus at 7q31.2 resides on chromosome 7, on the long (q) arm, within region 3, band 1, sub-band 2. Similarly, the tumor suppressor locus TP53 maps to 17p13.1, the adult β-globin locus (HBB) maps to 11p15.4, and the ABO glycosyltransferase blood-group locus maps to 9q34.2.

Meiotic Recombination, Centimorgans, and Physical Coordinates

After Thomas Hunt Morgan demonstrated in 1910–1911 that genes reside on chromosomes in Drosophila melanogaster, his student Alfred Sturtevant realized in 1913 that the proportion of recombinant offspring produced during meiosis could be used to construct a linear genetic map of loci along a chromosome. During prophase I of meiosis, paired maternal and paternal homologous chromosomes exchange segments via crossing over at chiasmata. The genetic map distance between two syntenic loci is measured in centimorgans (cM), where 1 cM corresponds to a 1% (0.01) probability of meiotic recombination per meiosis. Because double crossovers between distant loci restore parental allele combinations, Haldane's mapping function (d = −(1/2) ln(1 − 2θ)) and Kosambi's mapping function convert observed recombination fractions θ into additive genetic distances d in Morgans. In human pedigree linkage analysis, Newton Morton's LOD score (logarithm of the odds, base 10):

LOD(θ) = log₁₀ [ L(pedigree | linkage at fraction θ) / L(pedigree | free recombination θ = 0.50) ]

confirms autosomal linkage when LOD ≥ 3.0 (1,000:1 odds in favor of linkage).

Whereas genetic maps measure meiotic crossover frequency, physical maps specify linear distance in nucleotide base pairs (bp, kb, or Mb) along sequenced reference assemblies (GRCh38 or T2T-CHM13). In the human genome (~3,200 Mbp across a sex-averaged genetic map of ~3,500 cM), 1 cM averages roughly 1 Mbp (1,000,000 base pairs) of DNA, though local cM/Mb ratios vary across PRDM9-defined recombination hotspots and pericentromeric heterochromatin.

Homozygosity, Heterozygosity, Compound Heterozygosity, and Hemizygosity

Because somatic cells of diploid organisms carry two homologous sets of autosomes, every autosomal locus is present in two copies, defining four primary zygosity states:

• Homozygous (AA or aa): Both homologous chromosomes carry identical alleles at the locus (two wild-type reference alleles or two identical pathogenic alleles).
• Heterozygous (Aa): The two homologous chromosomes carry distinct alleles at the locus (one wild-type allele and one variant allele).
• Compound Heterozygous (a1a2): An individual carries two different pathogenic alleles at the same genetic locus—one on the maternal chromosome and another on the paternal chromosome (for example, p.Phe508del on one chromosome 7 homolog and p.Gly551Asp on the other at CFTR). Because neither allele produces a functional gene product, recessive disease ensues even though the individual is not homozygous for either single mutation.
• Hemizygous: Only a single copy of the locus is present in the diploid cell, as in heterogametic XY males for X- and Y-linked loci outside the pseudoautosomal regions (PAR1 and PAR2), where a single recessive pathogenic allele (DMD, F8) is expressed phenotypically.

Loss of Heterozygosity (LOH) in Cancer Genetics

In oncology, somatic Loss of Heterozygosity (LOH) at a tumor suppressor gene locus fulfills Alfred Knudson's (1971) two-hit model of tumorigenesis. When an individual inherits or somatically acquires a first-hit inactivating mutation in one allele of a tumor suppressor locus—such as RB1 at 13q14.2 in retinoblastoma, TP53 at 17p13.1, or APC at 5q22.2—the somatic cell remains normal because the wild-type allele is haplosufficient. Neoplastic transformation occurs when a somatic "second hit" eliminates the wild-type locus via chromosomal nondisjunction, interstitial deletion, or mitotic recombination (copy-neutral LOH).

Complex Traits, GWAS, eQTLs, and Locus Control Regions (LCRs)

Continuous polygenic phenotypes—such as stature, blood pressure, or crop yield—are governed by hundreds of Quantitative Trait Loci (QTLs) across the genome. Genome-Wide Association Studies (GWAS) scan millions of single-nucleotide polymorphisms (SNPs), utilizing linkage disequilibrium (LD) (D' and r²) between genotyped tag SNPs and causal variants. Because over 90% of complex-trait GWAS signals map to non-coding regulatory regions, functional genomics integrates GWAS with expression Quantitative Trait Loci (eQTLs)—loci where allelic variation correlates with mRNA abundance, divided into local cis-eQTLs (within 1 Mbp, acting allele-specifically on enhancers or promoters) and distant trans-eQTLs (acting via diffusible transcription factors).

In multigene families, clustered genes at a single chromosomal locus are often regulated jointly by a Locus Control Region (LCR). At the human β-globin locus spanning ~70 kb at 11p15.4, five functional globin genes lie in the linear 5' → 3' order of their developmental activation: embryonic HBE1 (ε), fetal HBG2 (Gγ) and HBG1 (Aγ), and adult HBD (δ) and HBB (β). Upstream (5') of HBE1 lies the β-globin LCR, composed of five erythroid-specific DNase I hypersensitive sites (HS1–HS5) that loop in 3D chromatin via CTCF and cohesin to contact the stage-appropriate globin gene promoter.

What is the difference between a gene, a locus, and an allele?

A locus is the physical or genetic coordinate ("address") on a chromosome (for example, human chromosome band 11p15.4 or nucleotide coordinates chr11:5,225,464–5,227,071). A gene is the functional transcriptional unit of DNA sequence occupying that locus (such as the HBB gene encoding the β-globin polypeptide chain). An allele is one of two or more specific nucleotide sequence versions of that gene found at that locus within a population (such as the wild-type HbA allele encoding glutamate at codon 6 versus the sickle-cell HbS allele encoding valine at codon 6).

How can a compound heterozygote develop a recessive genetic disorder without being homozygous?

Autosomal recessive disorders require loss of function in both copies of a gene locus. While consanguineous pedigrees often produce affected individuals who are homozygotes (carrying two identical copies of the same ancestral pathogenic mutation), outbred populations commonly produce affected compound heterozygotes who inherit one pathogenic allele from their mother and a different pathogenic allele at the same locus from their father (for example, one nonsense mutation and one missense mutation in CFTR). Because both homologous chromosomes carry defective versions of the same gene, no wild-type protein is synthesized, resulting in recessive disease.

Why does 1 centimorgan not always equal 1 million base pairs of DNA?

A centimorgan (cM) is a functional measure of meiotic crossover frequency (1% recombination per meiosis), whereas a megabase (Mb) is a physical count of 1,000,000 DNA nucleotide pairs. Although 1 cM averages roughly 1 Mb across the human genome as a whole, meiotic double-strand breaks catalyzed by the topoisomerase-like enzyme SPO11 are not distributed uniformly along chromosomes. Instead, crossovers are concentrated into narrow 1–2 kb euchromatic hotspots defined by PRDM9 histone H3K4me3/H3K36me3 methylation and are suppressed across pericentromeric heterochromatin and chromosomal inversions, causing local cM/Mb ratios to vary by more than 100-fold.

1. Sturtevant, A. H. (1913). "The linear arrangement of six sex-linked factors in Drosophila, as shown by their mode of association." Journal of Experimental Zoology, 14(1), 43–59. https://doi.org/10.1002/jez.1400140104
2. Morton, N. E. (1955). "Sequential tests for the detection of linkage." American Journal of Human Genetics, 7(3), 277–318. https://doi.org/10.1016/S0140-6736(00)89872-489872-4)
3. Knudson, A. G. (1971). "Mutation and cancer: statistical study of retinoblastoma." Proceedings of the National Academy of Sciences, 68(4), 820–823. https://doi.org/10.1073/pnas.68.4.820
4. GTEx Consortium. (2020). "The GTEx Consortium atlas of genetic regulatory effects across human tissues." Science, 369(6509), 1318–1330. https://doi.org/10.1126/science.aaz1776