Biology (A)

Linkage

Linkage refers to particular genetic position or loci, of alleles inherited together, suggesting that they are physically on the same chromosome, and located close together on that chromosome. A crossing-over event during prophase I of meiosis is rare between linked loci. Alleles for genes on different chromosomes are not linked; they sort independently (independent assortment) of each other during meiosis.

A gene is also said to be linked to a chromosome if it is physically located on that chromosome. For example, a gene (or loci) is said to be linked to the X-chromosome if it is physically located on the X-chromosome.

Linkage Maps

The frequency of recombination refers to the rate of crossing-over (recombination) events between two loci. This frequency can be used to estimate genetic distances between the two loci, and create a linkage map. In other words, the frequency can be used to estimate how close or how far apart the two loci are on the chromosome.

In the early 20^th century, Thomas Hunt Morgan demonstrated that the amount of crossing over between linked genes differs. This led to the idea that the frequency of crossover events would indicate the distance separating genes on a chromosome. Morgan's student, Alfred Sturtevant, developed the first genetic map, also called a linkage map.

Sturtevant proposed that the farther apart linked genes were on a chromosome, the greater the chance that non-sister chromatids would cross over in the region between the genes during meiosis. By determining the number of recombinants - offspring in which a cross-over event has occurred - it is possible to determine the approximate distance between the genes. This distance is called a genetic map unit (m.u.), or a centimorgan, and is defined as the distance between genes for which one product of meiosis in 100 products is a recombinant. So, a recombinant frequency of 1% (1 out of 100) is equivalent to 1 m.u. Loci with a recombinant frequency of 10% would be separated by 10 m.u. The recombination frequency will be 50% when two genes are widely separated on the same chromosome or are located on different chromosomes. This is the natural result of independent assortment. Linked genes have recombination frequencies less than 50%.

Determining recombination frequencies between genes located on the same chromosome allows a linkage map to be developed. Linkage mapping is critical for identifying the location of genes that cause genetic diseases.

Sequencing the genome of agriculturally important animals, such as cattle, can be important in the improvement of production. For more information on this aspect of modern genetics, please see http://www.physorg.com/news188148947.html