LINKAGE 



73 



which excludes the possibility of detecting any 

 linkage that is present, for obviously linkage cannot 





3 



4 



FIG. 29. Photograph of Pi (1 and 2) and f\ (3 and 4) birds in such a cross 

 as that of Fig. 30. The PI male is a standard figure. 



be seen unless at least two pairs of factors are studied 

 at the same time. The steady increase in the number 



