I02 Outline of Genetics 



individual. In the same way and with equal likelihood, 

 dwarf, red-flowered individuals may appear. 



This scheme serves to account for the occurrence of 

 the ^'exceptional" individuals in linkage cultures. The 

 whole phenomenon is known as crossing over. It has 

 been practically impossible to provide a direct demon- 

 stration that the chromosomes behave in exactly this 

 manner during the reduction division, but there has 

 accumulated an enormous mass of indirect evidence from 

 the breeding results to support this view. Evidently 

 the chromosome is not the indivisible unit in inheritance, 

 but is divisible according to a rather regular scheme. 

 Whole sections may be evenly exchanged between the 

 members of an allelomorphic pair of chromosomes. 



Once the phenomenon of crossing over had been 

 identified, investigation was undertaken to determine 

 the regularity and frequency of the phenomenon. It 

 was discovered that the amount of crossing over that 

 took place between a given pair of genes had a constant 

 value. For example, lo per cent of the crossing over 

 could be depended on to occur between T and R in every 

 experiment involving these two determiners. The 

 exact cross-over value is of course computed from the 

 breeding results obtained. In the present example, a 

 cross-over value of lo per cent between T and R would 

 work out as follows. In the reduction division (in the 

 Fi hybrid which results from tall red X dwarf white), 

 crossing over takes place in lo per cent of the cases, 

 while crossing over fails and the original linkage relation- 

 ships are maintained in 90 per cent of the cases. As a 

 result, four types of gametes are produced in the follow- 

 ing frequencies: 45 per cent TR, 45 per cent /r, 5 per 



