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. 
In the fourth group, where only three factors are 
known, it is found that there is no crossing over 
between them in the male, and only a very slight 
amount in the female. 
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 
1 Subject to certain variations which will be noted later. 
