io6 Outline of Genetics 



truth, this scheme of Morgan's has such an enormous 

 mass of data to support it that, for all practical purposes, 

 it may be regarded as an established fact. 



Cross-over values in the fruit fly may run as high as 80 per 

 cent, and in one of the related species much higher values have 

 been reported (Lancefield 4). Such a high cross-over value 

 seems rather surprising, for it represents a case where the cross- 

 overs are much more frequent than the non-cross-overs. It 

 should be reahzed that a cross-over value of 50 per cent, where 

 cross-overs and non-cross-overs are equally frequent, would give 

 exactly the same breeding results as if the two genes in question 

 were located on different chromosome pairs. Similarly, cross- 

 over values higher than 50 per cent would give the same breeding 

 results as though the linkage were reversed; that is, if M and N 

 were 80 units apart, the breeding results of a single experiment 

 involving M and N only would seem to indicate that M was linked 

 with n and m with N. It is evident, therefore, that these higher 

 cross-over values are computed from a considerable set of ex- 

 periments. Every newly discovered gene is carefully tested with 

 at least two other genes whose position is already known, and 

 thus the new gene is accurately placed on the chromosome. 



The cases that have just been discussed are known as single 

 cross-overs; the two chromosomes of the pair come to lie across 

 one another at a single point, and a single break with the subse- 

 quent rearrangement is sufficient to account for the results. In 

 view of the physical mechanism which seems to be responsible 

 for these cross-overs, it is not surprising to find that there may 

 sometimes occur double cross-overs. In these cases the two 

 chromosomes come to lie across one another at two points, and a 

 break takes place at each point, with the corresponding exchange 

 of chromosome regions. This amounts to an even exchange of 

 corresponding zones from the middles of the chromosomes, with 

 the two end zones on each chromosome remaining as before. Inas- 

 much as crossing over of any sort is detected only through its 

 effect on the breeding results, double crossing over can be demon- 

 strated only in experiments that involve observations on at least 

 three genes that are rather widely separated on the same chromo- 



