No. 632] SHORTER ARTICLES AND DISCUSSION 267 



beyond very short distances cross-over percentages do not in- 

 crease in proportion to distance. The linkage group forms a 

 means of holding genes together, however distant they may be 

 from each other, so that, as one goes, all have a tendency to go. 

 The linkage map will give us a diagrammatic view of the rela- 

 tions to each other of the genes composing a linkage system. It 

 is based on the shorter observed cross-over percentages, or where 

 longer distances are used, they must be first corrected for double 

 and triple crossing-over. See in this connection the valuable 

 Table II. of Haldane (1919) which provides a ready means of 

 converting map distances into cross-over percentages or vice 

 versa, and so of predicting undetermined linkage relations. It is 

 based on a mathematical examination of the linkage system of 

 the first chromosome of Drosophila. A table of linkage strengths 

 will show us, without reference to distances involved, to what 

 extent the movements in gametogenesis of one gene are correlated 

 with those of any other gene. It is based on the unmodified 

 cross-over percentages observed, whether the map distances in- 

 volved are great or small. Linkage strengths can never exceed 

 50 on a scale of 50, 100 on a scale of 100, whereas map-distances 

 may be extended indefinitely with the discovery of new genes. 



W. E. Castle 



Harvard University 



LITERATURE CITED 



Castle, W. E. 



1919. Studies of Heredity in Rabbits. Rats and Mice. Carnegie Inst. 

 Wash., Publ. No. 288. 



Dunn, L. C. 



Linkage in Mice and Rats. (In press.) 

 Haldane, J. B. S. 



1919. The Combination of Linkage Values, and the Calculation of 

 Distances between the Loci of Linked Factors. Journal of 

 Genetics, 8, pp. 299-309. 

 Morgan, T. H., Bridges, C. B., and Sturtevant, A. H. 



1919. Contributions to the Genetics of Drosophila melanogaster. Car- 

 negie Inst, Wash., Publ. No. 278. 



IS THERE LINKAGE BETWEEN THE GENES FOR 



YELLOW AND FOR BLACK IN MICE! 

 In a recent number of this journal Dunn 1 has givn data 

 showing a deficiency of black young in a family of yellow mice. 



