LINKAGE OF POLYGENES 



large chromosomes from the B Pm Sb tester stock, and one repre- 

 sentative of each from the wild-type strain under test. The B Pin 

 class is in the same case for chromosomes X and II, but it has both 

 its representatives of chromosome III from the wild-type strain. If, 

 therefore, chromosome III from the wild strain differs from its mate, 

 or homologue, from the tester stock in any genes affecting hair 

 number, there must be a corresponding difference in the average 

 numbers of hairs shown by the B Pm Sb and B Pm classes in the 

 backcross. The only limitation is that completely dominant genes 

 from the wild-type strain wiU obviously have no effects which can 

 be detected in a backcross of this kind. Similar estimates of the 

 difference in chromosome III can be found by the comparisons 

 between the average hair numbers of the B and B Sb classes, the 

 Pm and Pm Sb classes and the + (wild-type) and Sb classes. The 

 average difference in hair number due to genes in chromosome III 

 can be found therefore as 



i {B Pm— B Pm Sb+ B— B Sb+ Pm— PmSb+ { + )— Sb) 



where B Pm stands for the average hair number of the B Pm class 

 and so on. 



The effects of the other two chromosomes can be found similarly 

 as: — 



X chromosome J {Pm Sb— B Pm Sb-\- Pm — B Pm -j- Sb— BSb 



+ {+)-B) 

 II chromosome I {B Sb— B Pm Sb-^ B— B Pm+ Sb— Pm Sb 



+ (+)-Pm). 



The excesses of the three chromosomes of the two strains, S and O, 

 over the marked homologues from the tester, arrived at in this way, 

 are shown in the lower part of the Table. 



The differences between the chromosomes of either wild-type 

 stock and those of the tester might be influenced by a secondary 

 action of the marker genes, B, Pm and Sb. Other observations on 

 the effects of these genes would not lead us to expect such secondary 

 action, and indeed we can rule it out decisively in another way. 

 The S stock has an unmarked X chromosome which, so far as this 

 experiment goes, gives rise to the production of 2-66 more hairs 

 than the tester X chromosome marked by the gene B. The X chromo- 



75 



