Enzyme mapping phenotyping

LocusLink is an NCBI project to link information applicable to specific genetic loci from several disparate databases. Information maintained by LocusLink includes official nomenclature, aliases, sequence accessions (particularly RefSeq accessions), phenotypes. Enzyme Commission numbers, map information, and Mendelian Inheritance in Man numbers. Each locus is assigned a unique identification number, which additional databases can then reference. LocusLink is described in greater detail in Chapter 7. 

If the diploid has non-mutant (wild-type) phenotype, and it can be shown that they map at different positions (random spore analysis is most often used) then the lesions are in different genes. Such genes are named by adding a number after the letter code, e.g. his, hisl. Different genes which influence the same phenotype often produce proteins controlling for example a biosynthetic pathway. In this way the pathway and enzymes involved, in for example, histidine biosynthesis were elucidated. 

A similar collection of purine auxotrophs, with corresponding linkage groups and map positions, has been found in Escherichia coli [18]. Direct determination of corresponding enzyme deficiencies has not been made in most cases, but by phenotypic inference and map positions, a unified nomenclature of the E. coli genes has been made to correspond to that of the Salmonella data of Table I [19]. Essentially, there is a close correspondence between the two systems. The major difference is an apparent absence of purl mutants in E. coli and a closer linkage of purG to the gua operon. 

Cell fusion has also been used extensively in human chromosome mapping. The heterokaryons resulting from the fusion of human and mouse cells are genetically unstable and tend to lose human chromosomes randomly. This eventually gives rise to a mixed population of stable hybrids from which individual cell clones can be isolated. Many of these clones may contain single human chromosomes. It is the association of a particular chromosome in an isolated cell clone with a selected measurable phenotypic characteristic such as an enzyme activity that... 

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