fluoresecence in situ hybridisation (FISH) techniques (sect 10.1.4)Other maps have been obtained by mapping neutral chromosome breakpoints (using translocation and deletion hybrids; section 10.1.2), which is a more refined technique of physical mapping than that of using somatic cell hybrids and use only part of a particular chromosome. Translocation hybrids and deletion hybrids are made using by using donor human cells that have a chromosomal translocation or deletion. To be useful, the hybrids must lack the normal nomolog of the chromosome of interest. Such hybrids can be used for chromosomal mapping of a human sequenced tagged site or biochemical marker (fig 10.2). They are especially useful for defining the sequences removed by microdeletions, by segregating the deletion carrying chromosome away from it’s normal homolog. Alternatively, physical maps can be obtained by mapping artificial chromosome breakpoints using radiation hybrids (10.1.3). These are the most valuable hybrids for gene mapping (Walter et al., 1994). Donor cells are subjected to a lethal dose of radiation which fragments their chromosomes. The average size of a fragment is a function of the dose of radiation. After irradiation the donor cells are fused with recipient cells of a different species. A selection system is used to pick out the recipient cells that have taken up some of the donor chromosome fragments (box 10.1). These cells are useful for mapping in so faras they have taken up a random setn of other chromosome fragments from the donor, aswell as the selected fragment. Stably incorporated donor fragments are either intergrated into rodent chromosomes or are assembled into novel human minichromosomes formed around fragments containing a functional centromere. Although this procedure was first proposed by Goss and Harris in 1975, it was not used seriously until 1990, when hybrids were constructed using irradiated monochromosomal hybrid cells as donors ...
