Chromosome mapping

 A map is a line diagram consists of three things; the components, their relative position, and relative distance between them. Similarly, a chromosome map is a line diagram consists of the genes, their relative position, and the distance between these genes. The process of assigning genes on a chromosome is called chromosome mapping. Scientists use several methods to map genes to the appropriate locations. These methods include family studies, somatic cell genetic methods, cytogenetic techniques, and gene dosage studies. Family studies are used to determine whether two different genes are linked close together on a chromosome. If these genes are linked, it means they are close together on the same chromosome. Additionally, the frequency with which the genes are linked is determined by recombination events (crossing over of the chromosomes during meiosis) between known locations or markers and determines the linear order or genetic distance. Because the frequency of crossing over between any two linked genes is proportional to the chromosomal distance between them, crossing over frequencies are used to construct genetic, or linkage, maps of genes on chromosomes. The unit used to measure the distance between genes is called map unit or centimorgan (cM), to honor Geneticist T. H. Morgan. One map unit or cM equals 1% recombination frequency.

Before learning chromosome mapping, we need to understand the two-point test cross and the three-point test cross. A two-point test cross is a cross between a dihybrid and its recessive parent. A three-point test cross is a cross between a trihybrid and its recessive parent. In a three-point test cross, there can be a single cross-over and double cross over leading to the formation of eight phenotypic classes. A three-point test cross (involving three genes) gives us information regarding relative distances between the genes and tells us the linear order in which these genes are present on the chromosome. Three-point the cross is most preferred for chromosome mapping.