370 LINKAGE PHENOMENA 



winged gray-bodied hybrid male flies from the first type 

 of cross, where the two dominant characters (i.e., Gray 

 and Long) came from the same parent, are mated to 

 double recessive females (i.e., black vestigial) there will 

 be just two types of offspring; viz., Gray Long and black 

 vestigial like their grandparents. If free assortment had 

 occurred there would have been in addition recombina- 

 tion classes: viz., Gray vestigial and black Long and all 

 four classes in equal numbers, as was explained in a 

 preceding paragraph for the pea cross. 



If the second (2) type of gray long hybrid males are 

 crossed to black vestigial females they produce also only 

 two classes of offspring: viz.. Gray vestigial and black 

 Long like their grandparents and no recombination 

 classes. 



In both experiments there is a total failure to produce 

 the recombination classes to be expected according to 

 the law of free assortment. In other words, the charac- 

 ters that went into the cross together have stayed to- 

 gether or show complete linkage. The accompanying 

 diagram and those in the next paragraph show why these 

 results are obtained. 



Sperms of Oie two sorts 

 of gray long hybrids. 



Case > Gray Long sperm X black vestigial egg — Gray Long F^ 



(1) \ black vestigial sperm X " " " = black vestigial F, 



Case ) Gray vestigial sperm X " " " = Gray vestigial F^ 



(2) ) black Long sperm X " " " = black Long F^, 



The Segregation of Linked Genes. Since it has 

 already been shown that genes which are in different 

 chromosome-pairs assort independently it is evident that 

 characters which do not assort freely must be in the same 

 chromosome-pair. The accompanying diagrams (Fig. 

 107a and b) show the locations of the genes for vestigial 

 wing and black body color in the second chromosome-pair 



