gene mapping

History of genetics The history of genetics started with the work of the Augustinian friar Gregor Johann Mendel. His work on pea plants, published in 1866, described what came to be known as Mendelian Inheritance. In the centuries before—and for several decades after—Mendel's work, a wide variety of theories of heredity proliferated. 1900 marked the "rediscovery of Mendel" by Hugo de Vries, Carl Correns and Erich von Tschermak, and by 1915 the basic principles of Mendelian genetics had been applied to a wide variety of organisms—most notably the fruit fly Drosophila melanogaster. Led by Thomas Hunt Morgan and his fellow "drosophilists", geneticists developed the Mendelian, which was widely accepted by 1925. Alongside experimental work, mathematicians developed the statistical framework of population genetics, bring genetical explanations into the study of evolution. With the basic patterns of genetic inheritance established, many biologists turned to investigations of the physical nature of the gene. In the 1940s and early 1950s, experiments pointed to DNA as the portion of chromosomes (and perhaps other nucleoproteins) that held genes. A focus on new model organisms such as viruses and bacteria, along with the discovery of the double helical structure of DNA in 1953, marked the transition to the era of molecular genetics. In the following years, chemists developed techniques for sequencing both nucleic acids and proteins, while others worked out the relationship between the two forms of biological molecules: the genetic code. The regulation of gene expression became a central issue in the 1960s; by the 1970s gene expression could be controlled and manipulated through genetic engineering. In the last decades of the 20th century, many biologists focused on large-scale genetics projects, sequencing entire genomes. Gene mapping Gene mapping" refers to the mapping of genes to specific locations on chromosomes.  It is a critical step in the understanding of genetic diseases. Gene mapping, also called genome mapping, is the creation of a genetic map assigning DNA fragments to chromosomes. When a genome is first investigated, this map is nonexistent. The map improves with the scientific progress and is perfect when the genomic DNA sequencing of the species has been completed. During this process, and for the investigation of differences in strain, the fragments are identified by small tags. These may be genetic markers (PCR products) or the unique sequence-dependent pattern of DNA-cutting enzymes. The ordering is derived from genetic observations (recombinant frequency) for these markers or in the second case from a computational integration of the fingerprinting data. The term "mapping" is used in two different but related contexts. Two different ways of mapping are distinguished. Genetic mapping uses classical genetic techniques (e.g. pedigree analysis or breeding experiments) to determine sequence features within a genome. Using modern molecular biology techniques for the same purpose is usually referred to as physical mapping.   There are two types of gene mapping: Genetic Mapping - using linkage analysis to determine the relative position between two genes on a chromosome. Physical Mapping - using all available techniques or information to determine the absolute position of a gene on a chromosome. The ultimate goal of gene mapping is to clone genes, especially disease genes.  Once a gene is cloned, we can determine its DNA sequence and study its protein product.  For example, cystic fibrosis (CF) is the most common lethal inherited disease in the United States.  Linkage analysis The genetic mapping is based on the linkage between "loci" (locations of genes).  If two loci are usually inherited together, they are said to be "linked".  Two loci on different chromosomes are not linked, because they are usually separated by independent assortment. A locus (singular of loci) may have different sequences, referred to as alleles.  Consider two loci A and B, each having two alleles (one from mother, another from father).  A1 and A2 are the two alleles of locus A ; B1 and B2 are the two alleles of locus B.  Initially, A1 and B1 are located on the same chromosome.  A2 and B2 are located on a different chromosome. Figure 10-A-1.  Illustration of recombination between two loci A and B.  (a) Two pairs of sister chromatids align during meiosis.  A1 and B1 are located on the same chromosome.  A2 and B2 are located on a different chromosome. (b) DNA crossover leads to recombination if the chiasma is located between the two loci.  (c) DNA crossover does not lead to recombination if the chiasma is not located between the two loci. The DNA crossover may cause recombination of loci A and B.  Namely, A1 and B2 (or A2 and B1) are located on the same chromosome.  The recombination frequency depends on the distance between the two loci and the position of crossover (the chiasma).  The closer they are, the less likely the recombination will occur, because recombination occurs only when the chiasma is located between the two loci. To apply this basic principle to map a disease gene, we need to analyze the pedigree and estimate recombination frequency. Physical Mapping In physical mapping, the DNA is cut by a restriction enzyme. Once cut, the DNA fragments are separated by electrophoresis. The resulting pattern of DNA migration (i.e., its genetic fingerprint) is used to identify what stretch of DNA is in the clone. By analysing the fingerprints, contigs are assembled by automated (FPC) or manual means (Pathfinders) into overlapping DNA stretches. Now a good choice of clones can be made to efficiently sequence the clones to determine the DNA sequence of the organism under study (seed picking). Macrorestriction is a type of physical mapping wherein the high molecular weight DNA is digested with a restriction enzyme having a low number of restriction sites. There are alternative ways to determine how DNA in a group of clones overlap without completely sequencing the clones. Once the map is determined, the clones can be used as a resource to efficiently contain large stretches of the genome. This type of mapping is more accurate than genetic maps. Genes can be mapped prior to the complete sequencing by independent approaches like in situ hybridization. Disease-association The process to identify a genetic element that signs responsible for a disease is also referred to as "mapping". If the locus in which the search is performed is already considerably constrained, the search is called the "fine-mapping" of a gene. This information is derived from the investigation of disease-manifestations in large families (Genetic linkage) or from populations-based genetic association studies. Molecular genetics unit 1 3