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Home >>Molecular Maps of Animal Genome >>Example of RH (Radiation Hybrid) Mapping
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Example of RH (Radiation Hybrid) Mapping - In a recent experiment, Cox et al. (1991) used a chinese hamster human somatic cell hybrid (CHG3) containing a single copy of human chromosome 21. The cell line was exposed to 8000 rad of X-rays, fragmenting human chromosome 21 giving a mean of five pieces of this chromosome per cell.

Often these pieces were available as translocations with chinese hamster fragments, due to breakage and fusion. A dose of 8000 rad being lethal, irradiated cells were rescued by fusion with non-irradiated hamster recipient cells and using a medium that selects the hybrid cells.

These hybrid clones were assayed for retention of 14 markers of chromosome 21. Since markers were used for Southern hybridization, retention of more than one fragments in a hybrid created difficulties.

For instance, if two markers hybridize in a hybrid, one does not know whether there are two fragments or only one fragment with two marker.

However, assuming that retention of one fragment is independent of retention of the other, following relationship was used for estimation c, breakage frequency’Q' (analogous to recombination frequency in meiosis):
Q = {(A + B-) + (A - B+)} / {T (RA + RB - 2RARB)}

where (A +B-) is observed number of hybrid clones retaining marker

A , but not B; (A - B+) is the observed number of hybrid clones retaining marker B, but not A; T, is the total number of hybrids analysed; RA is the fraction of hybrid clones analysed for A and retain A; RB is the fraction of hybrid clones analysed for B and retained B. A 'Q’ value - of zero means two markers are never broken and 0 = 1 means two markers are always broken and are thus not linked.

Necessary tests for significant linkage are also performed. A mapping function D = log (1 - 0) is used to calculate distance in centiRays (cR) analogous to centiMorgans, where 1 Cr8000 - means 1% frequency of breakage between markers on exposure to 8000 rad of X-rays.

Although for individual pairs of markers, 0 and D can be calculated manually as above, but for a large number of pairwise combinations, computer programmes are used.

 
 

 

 
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