68 MOLECULES, VIRUSES, AND BACTEMA 



If any given mutant is the same size as g, according to our hypothesis 

 this ratio will be 1. If, on the contrary, a mutant is smaller than g, this 

 ratio will be less than 1. A mutant larger than g yields a ratio greater 

 than 1. We can thus construct a size order for the mutants which give 

 recombination with other mutants, considering each mutant in turn as 

 the reference mutant. 



When this is done, one observes the orders shown in Table II. Note 



TABLE II 



The Ordering of Mutant Sites with Respect to their Size. 

 Reference mutant Size order of mutants 



m^ g<h<f <c<j <i 



m^ g<h<c <f <j <d 



m,, g<h<f<c <j <d 



nif g<c = h <f <j <i 



mj g<c <f <h<j <i 



nii g<f<i<i 



m(j c <h <d 



crosses of mutant 



by wild DNA g<c = / = h <i <d<i 



first of all that since c, f, and h cannot be distinguished from one an- 

 other unless greater precision can be obtained in the measurement of 

 recombination frequencies, we cannot expect to order them with re- 

 spect to each other. They form a single size class which is larger than 

 that of g. Note finally that in no instance can we order both d and i in 

 the same cross, and we cannot distinguish them from each other. We 

 have, then, the possibility of ordering four classes: g, chf, j, and d or i. 

 The orders agree in all cases, and this is probably more than a happy 

 accident. 



In setting up this model, we have considered two variables: the 

 distance between markers and the linear dimensions of these markers. 

 We have supposed that the donor DNA molecule is uniformly paired 

 with the corresponding molecule in the recipient cell's chromosome. 

 We have thereby eliminated pairing as a variable. Only an experimental 

 test of the model will show whether pairing is not in reality a variable 

 also. However, regardless of this question, it is clear that the position 

 of a mutant site with respect to the ends of a DNA molecule will very 

 much influence its recombination behavior: If the recipient cell con- 

 tains a site which is near one end of the molecule, recombination fre- 

 quency will be depressed, for the probability of a point of attack occur- 

 ring in the region between the site and the end of the molecule will be 



