Linkage and Genetic Mapping

Genes that lie near each other on a chromosome are inherited together more often than chance allows, and the rate at which crossing over separates them provides a ruler for ordering genes along the chromosome.

Definition

Genetic mapping is the ordering of genes and the estimation of distances between them along a chromosome from the frequency of recombinant offspring, with one map unit (centimorgan) corresponding to one percent recombination.

Scope

This topic covers genetic linkage and the departure from independent assortment, the measurement of recombination frequency, its conversion into map distance in centimorgans, two-point and three-point testcrosses, the detection of double crossovers and interference, and the construction of linear linkage maps. It treats mapping at the level of meiotic recombination in transmission crosses; physical and genomic mapping methods are covered under genomics.

Core questions

• How does recombination frequency between two loci translate into a genetic map distance?
• Why is recombination frequency a reliable distance measure only over short intervals?
• How does a three-point testcross establish gene order and detect double crossovers?
• What is interference, and what does it reveal about the placement of crossovers?

Key concepts

• Genetic linkage and recombinant versus parental offspring
• Recombination frequency and the centimorgan
• Two-point and three-point testcrosses
• Gene order and double crossovers
• Interference and the coefficient of coincidence

Mechanisms

Recombinant gametes arise from crossing over between non-sister chromatids of homologous chromosomes during meiotic prophase I; the probability that a crossover falls between two loci rises with the physical distance separating them, making recombination frequency an estimator of that distance.

Clinical relevance

Linkage analysis was the principal method used to localize human disease genes before whole-genome sequencing and remains central to family-based studies; the same recombination logic structures the genetic maps that anchor genome assemblies and breeding programs.

History

After Morgan observed that sex-linked genes in Drosophila violated independent assortment, his undergraduate Sturtevant reasoned in 1913 that recombination frequencies could order genes linearly, producing the first genetic map and establishing recombination as a measure of chromosomal distance.

Key figures

• Thomas Hunt Morgan
• Alfred Sturtevant
• John B. S. Haldane

Seminal works

• sturtevant1913

Frequently asked questions

What is a centimorgan?

A centimorgan is a unit of genetic distance equal to a one percent chance of recombination between two loci in a single generation; over short intervals it corresponds roughly to physical distance, but the relationship varies along the genome.

Why can recombination frequency never exceed fifty percent?

Genes far apart on a chromosome, or on different chromosomes, recombine as if assorting independently, which yields equal numbers of recombinant and parental gametes and caps the observed recombination frequency at fifty percent.