Chapter 14: Genetic Mapping in Eukaryotes

Genetic Mapping in Eukaryotes begins by defining syntenic genes and linkage groups, explaining how genes located on the same chromosome tend to be inherited together unless separated by crossing-over events during the four-chromatid stage of prophase I in meiosis. The narrative highlights foundational experiments by Thomas Hunt Morgan using Drosophila melanogaster to demonstrate X-linked inheritance and establish that the frequency of recombination reflects the distance between genes. Furthermore, it details the work of Curt Stern, Harriet Creighton, and Barbara McClintock, who utilized cytological markers to prove that genetic recombination results from the physical exchange of homologous chromosome segments. A significant portion of the text is dedicated to the methodology of constructing genetic maps, detailing how recombination frequencies derived from two-point and three-point testcrosses are converted into map units or centimorgans (cM). The summary explains the statistical analysis of progeny phenotypes, identifying parental and recombinant classes to determine linear gene order and relative distances. Complex mapping concepts are clarified, including the detection of double crossovers, the calculation of interference, and the coefficient of coincidence, which measures how one crossover event affects the probability of another occurring nearby. The text also discusses mapping functions designed to correct for multiple crossovers which can cause recombination frequency to underestimate true map distance. The distinction between genetic maps, based on recombination rates, and physical maps, based on DNA sequencing and base pairs, is analyzed, noting phenomena like recombination hotspots that cause discrepancies between the two. Furthermore, the chapter addresses the unique challenges of mapping the human genome, where controlled crosses are impossible, introducing the statistical method of Logarithm of Odds (LOD) scores for pedigree analysis. It concludes with an overview of modern molecular tools, such as Restriction Fragment Length Polymorphisms (RFLPs), Short Tandem Repeats (STRs), and Single Nucleotide Polymorphisms (SNPs), which serve as critical DNA markers for genome-wide screens and the identification of disease-associated loci in contexts like the Human Genome Project.