Genetic Linkage

Genetic linkage is the tendency of alleles at loci that lie close together on the same chromosome to be inherited together more often than expected by chance. It is the exception to Mendel's law of independent assortment and the property that makes it possible to map genes by following how often nearby markers travel together through families.

Definition

Genetic linkage is the co-inheritance of alleles at two or more loci on the same chromosome at a frequency greater than predicted by independent assortment, reflecting their physical proximity and the low chance that recombination separates them during meiosis.

Scope

The entry covers what linkage is, why it deviates from independent assortment, the recombination fraction that quantifies it, and the LOD-score framework used to test for linkage in human pedigrees. It treats linkage as a reference concept in transmission and medical genetics, not as a clinical test.

Core questions

• When do two loci assort independently, and when are they linked?
• How is the strength of linkage quantified?
• How can linkage between a marker and a disease locus localize a disease gene?

Key concepts

• Linkage versus independent assortment
• Recombination fraction (theta)
• Coupling (cis) and repulsion (trans) phase
• LOD score
• Linkage analysis in pedigrees
• Marker loci

Mechanisms

Mendel's law of independent assortment holds for loci on different chromosomes or far apart on the same chromosome, because the alleles they carry are sorted into gametes independently. When two loci sit close together on one chromosome, a crossover rarely falls between them, so the parental combinations of alleles are usually transmitted intact and recombinant combinations are rare. The recombination fraction (theta), the proportion of meioses that are recombinant between the loci, ranges from near 0 for tightly linked loci to 0.5 for unlinked loci. In human genetics, where controlled crosses are impossible, linkage is detected statistically: the LOD score introduced by Morton (1955) compares the likelihood of the family data under a given recombination fraction against the likelihood under no linkage, with a LOD of 3 conventionally taken as evidence of linkage.

Clinical relevance

Linkage between a genetic marker and a disease locus is the basis of family-based gene localization; tracking which marker alleles co-segregate with disease in a pedigree can point to the chromosomal region where a disease gene lies. This describes how research evidence for gene location is generated and is reference background, not a diagnostic or treatment recommendation.

History

Linkage was first recognized when crosses violated independent assortment, and Morgan's group interpreted it as the physical association of genes on chromosomes. Sturtevant (1913) turned the recombination frequencies between linked Drosophila factors into the first linear genetic map. For human pedigrees, where experimental crosses are not possible, Morton (1955) developed the sequential LOD-score test, which became the standard method for detecting linkage and, with molecular markers, for localizing human disease genes.

Key figures

• Thomas Hunt Morgan
• Alfred Sturtevant
• Newton Morton
• Jurg Ott

Seminal works

• sturtevant-1913
• morton-1955

Frequently asked questions

What recombination fraction means two loci are unlinked?

A recombination fraction of 0.5 indicates that recombinant and non-recombinant gametes are equally frequent, which is what independent assortment produces, so the loci are effectively unlinked.

What does a LOD score of 3 mean?

It corresponds to odds of 1000-to-1 in favour of linkage at the tested recombination fraction over no linkage, the conventional threshold for declaring significant linkage in a pedigree study.