Linkage 107 



some. As would be expected, double cross-overs are inucli less 

 frequent than single cross-overs. There have also been reported 

 very rare cases of triple cross-overs. 



The question arises whether the frequency of crossing over 

 depends strictly and only upon the real distances between the 

 genes on a chromosome. In the first place, it is theoretically 

 possible that certain regions of a chromosome may, owing per- 

 haps to physical peculiarities, lend themselves more readily to 

 crossing over than do other regions of the same chromosome. (A 

 suggestion to this effect appears in some of Muller's work, see 

 chapter on ''Mutation. ") This possibility is, of necessity, ignored 

 in the chromosome maps, which are constructed purely on the 

 practical assumption that crossing over is proportional to distance. 

 In the second place, it is recognized that certain special influences, 

 such as temperature and age of the organism, may modify the 

 normal frequency of crossing over. Dependable values on crossing 

 over are to be obtained, therefore, only when such conditions are 

 absolutely standardized (Sturtevant 6). Further, one of the 

 outstanding and unexplained peculiarities of this phenomenon is 

 that, in the fruit fly, crossing over takes place freely in the female, 

 while none whatsoever takes place in any of the chromosome pairs 

 of the male. It is interesting to note that in organisms where the 

 female is heterozygous for sex — female homozygous for sex in the 

 fruit fly — exactly the reverse is true, crossing over taking place in 

 the male but not in the female (Tanaka 7). In corn, where both 

 sexes are represented on the same individual, there is no such 

 restriction, for crossing over takes place with approximately 

 equal frequency in microsporogenesis as in megasporogenesis 

 (Emerson and Hutchinson 3). Finally, it has been demon- 

 strated that the presence of certain special genes will modify the 

 frequency of crossing over, so that it has been possible to isolate 

 races of the fruit fly in which an extremely high or an extremely 

 low rate of crossing over takes place (Sturtevant 6, Detlef- 



SEN l). 



