CHAPTER VIII 

 CROSSING OVER AND CHROMOSOMES 



There are several occasions in the maturation period 

 of the germ-cells when it would seem that there might be 

 an opportunity for an interchange between like chromo- 

 somes. Such an occasion might be found at the time 

 when the thin threads twist around each other; or it 

 might be found after fusion of the threads, or possibly 

 after a general breaking up of the chromosomes and 

 reunion of the pieces. Unfortunately the cytological 

 evidence does not furnish explicit information as to the 

 stage at which interchange takes place. 



It has also been suggested that crossing over may take 

 place at a still earlier stage in the germ-tract, i.e., long 

 before the time of maturation, even in the early embryo. 

 Fortunately, it has been possible to obtain critical genetic 

 evidence showing approximately the time when crossing 

 over takes place. This evidence was obtained by Plough in 

 his work on the influence of temperature on crossing over 

 in Drosophila melanogaster. 



The way in which Plough's experiment was carried 

 out was as follows : Females homozygous for three mutant 

 factors in the second chromosome, viz.^ black, purple, 

 curved, were mated to wild-type flies. Some of these 

 females were kept in an incubator, some in an ice-box, 

 and some were kept at room temperature; under one 

 or the other of these conditions they laid their eggs 

 which hatched and produced larvae and pupae and flies. The 

 daughters were then mated to black, purple, curved males, 

 and remained under the same temperature conditions until 

 their offspring hatched. It was found that there was 

 more crossing over in the offspring of the pairs kept at a 

 high and at a low temperature than in those kept at room 



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