Chapter 12 

 CHROMOSOME MAPS 



Linkage Groups 



In the last chapter it was shown that a number of hnked genes 

 could be mapped on the assumptions that the amount of cross- 

 ing over is proportional to the distance between the genes on 

 the chromosome and that a crossover may occur with equal 

 freedom at any place on a chromosome except the region imme- 

 diately adjacent to another crossover. On this basis genes have 

 been mapped in a number of organisms. 



A group of genes showing linkage relationships with one an- 

 other genetically is a linkage group; and since all the genes in 

 one linkage group are believed to be located on the same chro- 

 mosome, the number of linkage groups should correspond with 

 the number of chromosomes observed cytologically in the same 

 species. It has been found to be true in several organisms in 

 which the number of genes discovered and located is sufficiently 

 large for the number of linkage groups to be determined with 

 reasonable assurance. To be sure of the correspondence be- 

 tween the number of linkage groups and the number of chromo- 

 somes, a large number of genes must be used in most organ- 

 isms for, if only a few are found, it is highly probable that 

 some would be so far removed from the others on the same 

 chromosome that they would give the same genetic ratios as 

 they would if they were on a separate chromosome. To be 

 certain of the number of linkage groups, then, ordinarily a large 

 number of genes must be discovered. It would not, of course, 

 be necessary for such an organism as Drosophila melanogaster 

 in which there is no crossing over in one sex, for one gene far 

 removed from the others could still be placed in its proper link- 

 age group by means of the 2:1:1:0 ratio obtained from 

 crosses in the repulsion phase when there is complete linkage 

 in one sex. Linkage groups have been worked out carefully for 



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