Functional Positional

Functional cloning involves using a known gene sequence as a probe to look for new gene sequences that may have similar functions, based on the similarity of sequence. Using this method of analysis, researchers can search through entire genomes to find genes of interest without having prior knowledge of their location. The genes, once putatively identified, can then be tested to see if their function is in fact similar to the initial gene of interest. This technique is useful when a given pathway or enzyme is present in one organism, and one wishes to identify the presence of a similar pathway or enzyme in a different organism.Phage expression libraries can be screened using probes designed based on the gene of interest. Functional cloning versus positional cloning“One way to conceptualize the difference between functional and positional cloning is to consider methods for looking up words. Functional cloning is akin to using a thesaurus to look up a known word and thereby to select new words with related meanings (or functions). Positional cloning is similar to reading though a specific page of a dictionary, browsing for interesting words of any meaning located on that particular page. In the first case, words and genes are selected for their function, be it in a sentence or in a cell. In the second case, words and genes are selected for position regardless of their function.”[2] Positional cloning is the method of finding a gene without any knowledge of the protein it encodes. The first human disease gene identified by positional cloning was one for chronic granulomatous disease. 1986: First Time Gene Positionally Cloned.Perturbations in more than one gene can cause the CGD phenotype, characterized by the presence of inflamed lesions containing phagocytic immune cells. A group from Boston Children's Hospital isolated the CGD gene on Xp21. The genes for Duchenne muscular dystrophy and retinoblastoma followed quicklygranulomatous disease, chronic, x-linked; cgd cytochrome b-negative granulomatous disease, chronic, xlinked,chronic granulomatous disease, x-linked,cytochrome b-positive granulomatous disease, chronic, x-linked, included granulomatous disease, chronic, x-linked, variant, included chronic granulomatous disease, atypical, included Gene map locus Xp21.1 MAPPING Francke et al. (1985) studied a male patient with 3 X-linked disorders: chronic granulomatous disease with cytochrome b(-245) deficiency and McLeod red cell phenotype (314850), Duchenne muscular dystrophy (310200), and retinitis pigmentosa (RP3; 300389). A subtle interstitial deletion of part of Xp21 was demonstrated as the presumed basis of this 'contiguous gene syndrome.' The close clustering of CGD, DMD and RP suggested by these findings was inconsistent with separate linkage data (see HISTORY and Densen et al., 1981), which indicated that McLeod and CGD are close to Xg and that DMD and RP are far from Xg. Francke et al. (1985) suggested that the deletion may contain a single defect affecting perhaps a cell membrane component which underlies all 3 disorders. Using a method for cloning the specific DNA fragment absent in patients homozygous or hemizygous for chromosomal deletions, Kunkel et al. (1985) confirmed a minute interstitial deletion of Xp21 in the patient reported by Francke et al. (1985). Using cloned, polymorphic DNA probes, Baehner et al. (1986) mapped CGD to Xp21.2-p21.1, proximal to DMD. CGD lies in a region of Xp that appears to have more recombination than anticipated on the basis of physical distance between markers. This linkage assignment is inconsistent with the linkage to Xg, but entirely consistent with the findings in the boy reported by Francke et al. (1985) with an interstitial deletion of Xp21, The earlier data on linkage to Xg were apparently in error