LIMITATION OF THE LINKAGE GROUPS 137 



of intermediate genes shows their true relation. This will 

 be especially the case when crossing over occurs in both 

 sexes ; when it occurs only in one sex, the linkage relations 

 are more quickly determined. Moreover, in some cases 

 where several genes are known the mutant characters 

 have not yet been tested out against each other but against 

 different ones. Such information does not furnish the 

 data that are needed. 





a 



b 



Fia. 55. — Haploid group of chromosomes of the silkworm moth (Yatsu) a. Haploid 

 group of chromosomes of mouse (Yocom) b. Haploid group of chromosomes of man 

 (Guyer), c and (von Winniwarter) d. 



There are several forms in which there are two or 

 more chromosomes that come together in a group at the 

 time of segregation and move collectively to one pole. 

 Such groups should be expected to count as a single chro- 

 mosome so far as segregation is concerned, although the 

 crossing over relations may turn out to be something 

 different from anything as yet known. 



