78 CELL HEREDITY 



Chapter 4 some cytogenetic tests will be described which demonstrate 

 this correspondence in even more detail. Cytology and genetics have 

 become a powerful team in the study of heredity and have made possible 

 a much more direct and rapid analysis than formal genetics alone al- 

 lows. The more recent recognition of nucleic acids as genetic material 

 has permitted the further combination of chemical with genetic tech- 

 niques. In all this the process of genetic exchange has been a most 

 powerful tool; it has even been used for the analysis of the molecular 

 fine structure of DNA, a feat as yet impossible by the use of chemical 

 techniques alone (see Chapter 6). 



One reason why recombination analysis has been so successful is that 

 it allows the location of hereditary factors on genetic maps. When the 

 factors are chromosomal genes, it is the chromosome that is mapped. In 

 those cases where the products of a single meiosis can be collected for 

 study, even the centromere may be given its position. In Neurospora, 

 the frequency of second-division segregation measures the distance be- 

 tween the gene observed and the centromere because such segregations 

 result from crossing over between them. To convert these frequencies 

 into map units, they must be divided in half because each crossing 

 over, although it results in one second-division segregation, yields only 

 50 per cent recombination. In actual experiments, 12 per cent of 3725 

 asci showed second-division segregation of the mt gene, and 54 per cent 

 of 1263 asci showed second-division segregation of the alb gene. The 

 mt locus is then 6 units, and the alb locus 27 units, from the cen- 

 tromere. But from this alone we cannot decide whether mt and alb are 

 on the same or opposite arms of the chromosome. If the former were 

 true, the distance between mt and alb would be 21; if the latter were 

 true, the distance would be 33. This distance has already been esti- 

 mated by the frequency of recombinations between alb and mt, which 

 was 31 per cent. The close correspondence between 31 and 33 indicates 

 that the chromosome map should read: 



33 



LIMITATIONS IN MAPPING 



If, as genetic map construction assumes as a first approximation, 

 crossing over occurs with a constant chance per unit of chromosome 



