How to find distance between genes

Geneticists are frequently interested in knowing where a gene is located within the genome. There are many reasons for this interest. For example, in many well-studied genetic systems, including humans, an accurate genetic location makes it feasible to proceed with a molecular biological analysis of the gene and to eventually learn what protein the gene encodes. Also, knowledge of gene location is useful in constructing special strains for specific experimental purposes. Maps of the gene loci on chromosomes can be constructed by recombination analysis; mapping techniques will be discussed below. The recombinant frequency (RF) is different in crosses involving different linked heterozygous genes. However, for any one case the RF value is characteristic and experimentally reproducible. The RF for two loci is proportional to the physical distance between them on the chromosome. This makes sense because if two loci are far apart on a chromosome, we would expect that the proportion of meiocytes in which a crossover occurs between the loci would be greater than in cases in which two linked loci are very close together. (If we think of chiasmata as pellets fired from a shotgun, it is intuitive that there will be a greater chance of hitting a large target—a large chromosomal distance—than a small one.) Thus, by determining the frequency of recombinants, we should be able to obtain a measure of the map distance between the genes. In fact, one genetic map unit (m.u.) is defined as that distance between genes for which one product of meiosis out of 100 is recombinant. Put another way, a recombinant frequency of 1 percent is defined as 1 m.u., and an RF of 6 percent is defined as 6 m.u., and so on. A map unit is sometimes referred to as a centimorgan (cM) in honor of Thomas Hunt Morgan, a pioneer in the field of chromosome mapping. A direct consequence of the way map distance is measured is that if 5 map units (5 m.u.) separate the genes A and B whereas 3 m.u. separate the genes A and C, then B and C should be either 8 or 2 m.u. apart (Figure 5-10). Generally this is found to be the case; geneticists say that map distances are roughly “additive.” In other words, the RF analysis is compatible with the fact that genes are arranged in linear order on the chromosome. #geneMapping #chromosomeMapping #GeneDistance #CentiMorgans #mapUnits #genetics #alleles