CHAPTER VII 

 CROSSING OVER 



The correlative aspect of linkage is crossing over, and 

 inasmuch as it involves a change in the mechanism that 

 gives linkage, it is entitled to rank as one of the funda- 

 mental principles of heredity. 



In the illustration of complete linkage given in the 

 preceding chapter, the cases chosen were ones in which 

 entire chains of genes remained intact during the reduc- 

 tion divisions. The male of Drosophila exhibits this phe- 

 nomenon, as does also the female of the silk-worm moth. 

 On the other hand, there is an interchange of blocks of 

 genes between homologous pairs of chromosomes in other 

 cases, as in the females of Drosophila, in the males of 

 moths and fowls, and in both egg-cells and sperm-cells 

 of the plant Primula} This interchange is called crossing 

 over, and the evidence shows that it is not haphazard, but 

 gives numerical results of extraordinary constancy. A 

 few examples will suffice to illustrate crossing over. 



When a black fly with vestigial wings is crossed to 

 a wild-type (**gray'') fly with long wings (Fig. 33) the 

 offspring are, as we have already seen, ^'gray,'' long. 

 If one of the F^ females is back-crossed to a black ves- 

 tigial male there are four kinds of offspring produced, viz., 

 the two original combinations, black vestigial, and gray 

 long; and in addition two recombinations of these, viz., 

 black long, and gray vestigial. The two latter classes are 

 called the crossover classes, or more briefly, crossovers. 

 The i3ercentage of crossovers is definite for a given stock, 



^ Crossing over in both sexes in the rat has beem reported by Castle 

 and Wright, and in the male and female grasshopper by Nabours. 



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