92 AN INTRODUCTION TO MODERN GENETICS 



parts of chromosome with an average frequency of cross-over have it 

 reduced if translocated to a position with low cross-over.^ 



2. Maps Prepared on other Bases 



The map based on the cross-over values in the triploid shows the 

 genes arranged in the same order as in the diploid but with different 

 intervals.^ This modification of the ordinary cross-over values is prob- 

 ably due to varying amounts of pairing in prophase; in a triploid only 

 two threads are paired at any one point, the third chromosome being 

 left out of association at that part but pairing in some other region 

 with one of the sister chromosomes (p. 69). Crossing-over is only 

 possible in the paired regions, and the percentage of crossing-over 

 between two chromosomes in a region is therefore dependent on the 

 occurrence of pairing between these two out of the three chromosomes. 



Muller^ has prepared a map based on mutation frequency, arguing 

 that the chance of a mutation is dependent only on the number of 

 genes in a section of chromosome and that this number may be directly 

 proportional to the length of chromosome; this involves the further 

 assumption that the average mutation rate of the genes in a moderately 

 long section of chromosome is constant (cf. p. 380). Maps prepared on 

 this basis are slightly modified according as one considers mutations to 

 different allelomorphs of the same locus as being different, or whether 

 one simply measures the distance between two genes by the number of 

 loci known between them. The fullest and most recent maps of this 

 kind are based on the latter consideration and have been given by 

 Schweitzer.* 



3. Genetic and Environmental Effects on Crossing-over 



The amount of crossing-over is under genetic control. This is shown 

 most strikingly by the complete suppression of crossing-over in males 

 of Diptera and its general lowering in the heterogametic sex. Its sup- 

 pression in male Drosophila has been shown to depend on the develop- 

 ment of an abnormal method of chromosome pairing in meiosis. Genes 

 are also known which suppress or lower the frequency of crossing-over 

 in all the chromosomes of the fly which contained them.^ These factors 

 seem also to perform the suppression by some action on chromosome 



^ Offermann, Stone and MuUer 193 1. - Cf. Sansome and Philp 1932. 



' Muller and Painter 1932. * Schweitzer 1935. 



^ Gowen and Gowen 1922, 1933a; for a similar gene in maize, see Beadle 

 1933. 



