Linkage Groups in Human Beings 



195 



mosome IV. This region contained several 

 distinct bands and several finely dotted 

 bands. When the broken end of the X 

 chromosome was studied, it was found that 

 none of the bands to the right of the region 

 of white was missing. This region between 

 the translocation and the centromere of 

 chromosome IV must therefore represent a 

 left arm of chromosome IV which had pre- 

 viously not been observed (Fig. 59t>). It 

 had apparently been overlooked in previous 

 studies because the whole chromosome is 

 very small and because it is almost always 

 observed in the temporary aceto-carmine 

 mounts with which salivary gland chromo- 

 somes are frequently studied. To show 

 this arm, the smearing must be done with 

 great care, and Bridges's permanent method 

 should be used. 



Linkage Groups in Human Beings 



Extensive linkage maps have been pre- 

 pared for Drosophila melanogaster and Zea 

 mays, but almost nothing is known of link- 

 age in man. In most organisms in which a 

 number of genes have been discovered, these 

 genes are frequent enough for appropriate 

 crosses to be made to test the linkage rela- 

 tionships of a large number of them. In 

 man, however, most of the genes that have 

 been discovered are infrequnt in natural 

 populations ; the chance that two such genes 

 will be found in any one family is therefore 

 small. In man, too, the number of chro- 

 mosomes is considerably larger than in the 

 plants and animals in which linkage has 

 been studied intensively. For example, the 

 chance that two random genes would be on 

 the same chromosome is greater in Dro- 

 sophila melanogaster than in man, where the 







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