54 LINKAGE 



male, the amount of crossing over is different for 

 different factor combinations but, for any given com- 

 bination, it is not altered by the way in which the 

 factors entered the cross, and is, ordinarily,^ constant. 



Tests like the preceding ones for the second group 

 have been carried out for the third group, and give 

 the same kind of results. There is crossing over in 

 the female and no crossing over in the male. 



At present only two members of the fourth group 

 are known, and the phenomena of linkage have not 

 yet been studied in detail,, but it is probable that 

 there is no crossing over in one sex. 



In the first group (sex linked" characters) , a very 

 large amount of data has been collected. Here again 

 there is abundant evidence to show that crossing 

 over takes place in the female, but not in the male. 

 The curious fact also comes to light that no mutations 

 have been discovered in the Y chromosome, nor does 

 it contain any factors dominant to any known 

 mutant or normal factors in its mate, the X chromo- 

 some. Since the linkage of a considerable number 

 of factors in the X chromosome has been studied in 

 detail the evidence from this source best serves to 

 illustrate cases where the linkage is strong, where it 

 is moderate, and where it is weak. 



The body color called yellow and the eye color 

 white have been used in many experiments. If a 

 yellow white female is mated to a wild male (gray 

 red) (Fig. 23), the daughters are gray with red eyes 

 (like the fathers), but the sons are yellow white like 



^ Subject to certain variations which will be noted later. 



