42 MOLECULES, VBRUSES, AND BACTERLV 



a function of the length of the chromosome segment separating them. 

 Consequently, the distance between two linked genes can be measured 

 in terms of the frequency with which they are separated by crossing- 

 over. The genetic unit of length, or Morgan unit, is that distance 

 separating two points on a chromosome which recombine at meiosis 1 

 per cent of the time. 



Two difficulties are encountered in measuring with this genetic 

 ruler: 



1. If a chromosome is marked at several points by mutants A, B, C, 

 and D, the sum of the distances AB, BC, and CD is usually greater 

 than that measured by the frequency of recombination between A and 

 D. This is generally attributed to the occurrence of more than one cross- 

 over in the interval AD. If multiple cross-overs occur, and their num- 

 ber is odd, recombination of A and D will be observed. If, however, 

 the number is even, hnkage between A and D is maintained and the 

 resulting chromosome is scored as non-recombinant. Thus the total 

 number of cross-overs in the large interval is systematically under- 

 estimated. 



2. If a cross-over occurs in one region of a pair of chromosomes, 

 this will diminish the occurrence of additional cross-overs in the same 

 pair. This phenomenon, called interference, tends to diminish the 

 mapping errors due to multiple cross-overs. 



The question naturally arises: To what physical length does the 

 Morgan unit correspond? With the development of new Kinds of 

 genetic systems in which crosses involve pairing and hybridization of 

 DNA molecules, recombination of linked markers can be seen to be a 

 phenomenon extending down to the molecular scale ( Ephrussi-Taylor, 

 1951; Hershey and Rotman, 1948). Therefore, in terms of molecular 

 genetics, the question which we have just stated is equivalent to asking: 

 To what length of nucleotide sequence does the Morgan unit corre- 

 spond? If the Morgan unit could be measured by direct means, then the 

 length of sequence involved could be readily calculated, for the di- 

 mensions and structure of DNA are well established. This is not, how- 

 ever, possible. What we can do is estimate the total number of recom- 

 bination units and the total DNA content in a particular genome. From 

 the ratio of these two values, we obtain the length of sequence per 

 Morgan unit. When this is done for such unrelated genomes as those of 

 Aspergillus nidulans and bacteriophage T4, the Morgan unit in the 

 former is found to represent a linear sequence of 40,000 nucleotides, 

 while in the latter it represents only 1,000 (discussed in Pritchard, 

 1960 ) . It is clear that the Morgan unit is not the same in all organisms, 

 and one can conclude that frequency of crossing-over is not determined 

 by DNA structure and replication alone. 



