METHODS FOR TESTING LINKAGE 77 



between the sexes which they observed, however, is in accord with the known tendency 

 for crossing over in the female to be greater than that in the male. Assuming Slizynski's 

 counts in the male to be accurate but to give an underestimate of the frequency for 

 males and females combined, we may take 2.1 chiasmata per autosome as an approxi- 

 mation of the true value. Since one chiasma corresponds to 50 map units or centi- 

 morgans (cM), the average length of autosome can be taken as 105 cM, and the total 

 autosomal map length as 1,995 cM. Using these figures, we can calculate that the 

 total length of linkage map swept by these linkage testing stocks is 1 ,438 cM if the new 

 mutant is a dominant, and 1,390 cM if the new mutant is a recessive. By raising 100 

 test progeny from each mating, with these stocks one can test a new dominant against 

 about 72 per cent of the total autosomal map length and a new recessive against 70 per 

 cent. The sex chromosome is not included in these calculations because sex-linkage 

 will be obvious without recourse to these stocks. 



LINKAGE MAP OF THE MOUSE 



The known linkage map of the mouse is considerably longer than that of any other 

 mammal but it still falls far short of completeness. Nineteen of 20 possible linkage 

 groups are known, of which I, II, III, V, VII, VIII, IX, XI, XIII, and XX have been 

 shown by Slizynski 1218, 1219 to be on different chromosomes. Some of the remaining 

 groups are quite short and may appear to be independent of each other only because 

 they are located far enough apart on the same chromosome. Linkage group IV has 

 not been adequately tested against most of the other groups because the markers in this 

 group were, until recently, highly unsuitable for linkage tests. Ruby and jerker 

 appeared linked in earlier data but recent tabulation of unpublished data shows them to 

 recombine at random. They may therefore actually represent different linkage groups. 



The map in figure 1 8 summarizes the latest information on linkage in the mouse. 

 Most of it is based on published information but it also makes use of information from 

 personal communications and from the Mouse News Letter. The unpublished informa- 

 tion is used by permission of the authors. All mutants used in the map have been 

 described either in print or in the Mouse News Letter before August, 1960. Some of the 

 information on linkage has been supplied by personal communication as noted in the 

 bibliography. A few mutants for which there was preliminary indication of linkage, 

 but which are now thought to be extinct, have been omitted. Rodless retina, r, is prob- 

 ably extinct, but has been included because the evidence for linkage with silver, si, is 

 reasonably good, and because this was the linkage which first established linkage 

 group IV. 



For several reasons the distances between loci on the map are not to be taken too 

 literally. They represent many compromises, and recourse must be had to the 

 references cited for more exact information. The numbers on the map are recombina- 

 tion percentages, but not necessarily those between the two loci on opposite sides of the 

 numbers; it is not possible, without unduly complicating the map, to show which 



