GENES AND CHARACTERS 



309 



tera. It is easy to see that the separation of these chromosomes 

 during the reduction division might be accompanied ])y rupture 

 at some point of crossing and the formation of new chromosomes, 

 each made up of parts of the two original synaptic mates (Fig. 

 176). If the genes for Hnkod characters arc located on the opposite 

 sides of such a break, as indicated by the letters AB and ab, a 

 reassociation would result giving Ab and aB linkage in all cells 

 where crossing over has occurred. 



According to Morgan and his associates crossing over occurs 

 between many other linked characters in Drosophila, although in 

 varying percentages. It has not been observed in the male, but 



[Bl 



Fig. 176. — Diagram to illustrate the mechanism of crossing over. 



occurs ordinarily in a constant percentage of germ cells in the 

 female, and is independent of the way in which the characters 

 are at first combined. 



Multiple Crossing Over. Since the chromosomes during synap- 

 sis are twisted spirally al^out each other it is possible for a rupture 

 to occur at one point of crossing as easily as at another, and 

 perhaps at more than one point during the same cell division. 

 Such an interchange would result in new chromosomes made up 

 of alternate pieces of the two original synaptic mates. In the 

 case of a double crossover (Fig. 177) involving three genes, two 

 of the characters would remain linked as before, but the middle 

 one would be interchanged, so that different linkage groups would 

 be produced. Multiple crossing over would necessarily have a 

 different effect on the percentage of interchange between linked 

 characters, and in Drosophila it has been found adequate to 

 explain several departures from the expected Mendclian ratios. 



