Linkage Recombination Frequency Calculator

Recombination frequency is the proportion of offspring carrying a combination of alleles that neither parent transmitted intact, produced by crossing over between two loci during meiosis. It is measured from a test cross, where the doubly recessive parent contributes only one gamete type and offspring phenotypes therefore report the other parent's gametes directly. The value is bounded between 0, meaning complete linkage, and 0.5, the signature of independence, meaning either separate chromosomes or loci far enough apart that a crossover between them is effectively certain. One percent recombination is defined as one centiMorgan. Across the human genome the map totals roughly 3,500 cM over about 3.1 billion base pairs, so a centiMorgan averages near one megabase, though the local ratio varies enormously: recombination is suppressed near centromeres and concentrated in narrow hotspots. The reason mapping functions exist is that recombination frequency systematically understates true distance beyond about 10 to 15 cM. A double crossover between two markers restores the parental arrangement and goes uncounted, so the observed value saturates at 0.5 no matter how far apart the loci actually are. The Haldane function corrects for this assuming crossovers follow a Poisson process with no interference; the Kosambi function assumes positive interference and generally fits real organisms better. Only after correction do distances add up reliably across adjacent intervals. Sample size is the other constraint: a hundred offspring cannot reliably separate a recombination frequency of 45 percent from true independence, which is why the chi-square test and the LOD score matter, with a LOD of 3 the conventional threshold for declaring linkage.

The phenomenon was noticed before it was understood. In 1905 William Bateson and Reginald Punnett reported sweet pea crosses in which flower colour and pollen shape failed to assort independently, an effect they called coupling and repulsion without being able to explain it. The explanation came from Thomas Hunt Morgan's laboratory at Columbia University, where work on Drosophila led him to propose in 1911 that homologous chromosomes physically exchange segments, and that genes lying closer together are separated by such exchanges less often. In 1913 Alfred Sturtevant, then an undergraduate in that laboratory, used pairwise recombination percentages to order six sex-linked genes along the Drosophila X chromosome, producing the first genetic map. Morgan received the Nobel Prize in Physiology or Medicine in 1933, and the centiMorgan carries his name. The mathematics followed. J. B. S. Haldane published his mapping function in 1919, treating crossovers as Poisson-distributed and independent, and D. D. Kosambi published an alternative in 1944 that accommodated crossover interference. Human genetics posed a harder problem because controlled crosses are impossible and pedigrees are small, so Newton Morton introduced the LOD score in 1955, a sequential likelihood test whose values can be summed across families; his framework is the origin of the familiar threshold of 3. Practical human mapping became feasible after David Botstein, Raymond White, Mark Skolnick and Ronald Davis proposed in 1980 that restriction fragment length polymorphisms could serve as abundant genetic markers, an approach that led to the linkage-based identification of the Huntington disease and cystic fibrosis loci.