A report on Genetic linkage and Centimorgan

Thomas Hunt Morgan's Drosophila melanogaster genetic linkage map. This was the first successful gene mapping work and provides important evidence for the chromosome theory of inheritance. The map shows the relative positions of alleles on the second Drosophila chromosome. The distances between the genes (centimorgans) are equal to the percentages of chromosomal crossover events that occur between different alleles.

Pedigree illustrating Parametric Linkage Analysis

In genetics, a centimorgan (abbreviated cM) or map unit (m.u.) is a unit for measuring genetic linkage.

- Centimorgan

The typical unit of genetic linkage is the centimorgan (cM).

- Genetic linkage

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Crossing over occurs between prophase I and metaphase I and is the process where two homologous non-sister chromatids pair up with each other and exchange different segments of genetic material to form two recombinant chromosome sister chromatids. It can also happen during mitotic division, which may result in loss of heterozygosity. Crossing over is important for the normal segregation of chromosomes during meiosis. Crossing over also accounts for genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical. So, when the chromosomes go on to meiosis II and separate, some of the daughter cells receive daughter chromosomes with recombined alleles. Due to this genetic recombination, the offspring have a different set of alleles and genes than their parents do. In the diagram, genes B and b are crossed over with each other, making the resulting recombinants after meiosis Ab, AB, ab, and aB.

Chromosomal crossover

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Exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes.

Thomas Hunt Morgan's illustration of crossing over (1916)

A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with a homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.

The difference between gene conversion and chromosomal crossover.

For fixed set of genetic and environmental conditions, recombination in a particular region of a linkage structure (chromosome) tends to be constant and the same is then true for the crossing-over value which is used in the production of genetic maps.

This leads to the notion of "genetic distance", which is a measure of recombination frequency averaged over a (suitably large) sample of pedigrees.

Thomas Hunt Morgan

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American evolutionary biologist, geneticist, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that the chromosome plays in heredity.

In a typical Drosophila genetics experiment, male and female flies with known phenotypes are put in a jar to mate; females must be virgins. Eggs are laid in porridge which the larvae feed on; when the life cycle is complete, the progeny are scored for inheritance of the trait of interest.

Sex linked inheritance of the white eyed mutation.

Morgan's illustration of crossing over, from his 1916 A Critique of the Theory of Evolution

The observation of a miniature-wing mutant, which was also on the sex chromosome but sometimes sorted independently to the white-eye mutation, led Morgan to the idea of genetic linkage and to hypothesize the phenomenon of crossing over.

The later English geneticist J. B. S. Haldane suggested that the unit of measurement for linkage be called the morgan.