70 MOLECULES, VIRUSES, AND BACTERDS. 



tion event is scored in a given cross: the transfer of donor specificity to 

 the recipient-cell genome. The reverse transfer is not detected because 

 if it occurs, the resultant molecule cannot form the genome of a viable 

 cell. Thus, in order to study such an event, we must reverse the cross 

 by making a DNA from the recipient cell and transform the donor of 

 the first cross. A second consequence of the incompleteness of the 

 "male" element of crosses in transformation is that, because this ele- 

 ment is a single DNA molecule, a sharp upper limit is imposed on the 

 length of the paired structures that give rise to recombinants. Both of 

 these factors have served in the building of a model. The first factor is 

 the very reason for the detection of the unequal frequencies with which 

 the two kinds of recombinations give rise to wild-type recombinants, 

 for each kind of recombination event is obtained in pure form in one of 

 the two reciprocal transformations possible between two marked 

 strains. The absence of symmetry of the crosses of the amylomaltase 

 mutants is the restriction that indicates absence of proportionality be- 

 tween map distance and recombination frequency. The second factor- 

 namely, a limitation of the size of the paired structures— justifies mak- 

 ing a simple model in which pairing is not a variable. Experiments will 

 reveal whether this simplification is valid. 



In crosses involving two complete genomes, as in Aspergillus or 

 phage, selected recombinants arise from what may formally be con- 

 sidered the same two kinds of recombination events mentioned above. 

 For example, in cross A shown in Figure 2 the selected recombinants 

 that are wild type for adenine are formed by two basic events. 



1. The new chromatid synthesized along parent 1 may switch over 

 to parent 2, copy the normal sequence opposite the adn site, and return, 

 or not, to copy again along the parent 1 chromosome. 



2. The chromatid synthesized along the parent 2 chromosome may 

 switch over and copy the normal sequence opposite the adia site, and 

 return, or not, to copy again along the parent 2 chromosome. 



Translated into the terms used for transformation: In the first in- 

 stance the parent 1 chromatid is serving as the "recipient" and the par- 

 ent 2 as the "donor"; in the second instance parent 2 is the "recipient" 

 and parent 1 the "donor." If we are to examine the applicability of the 

 transformation model to other genetic systems, we must be able to 

 recognize these two categories of events. A step in this direction can be 

 made if additional markers are present outside the locus of the re- 

 combining alleles. The selected recombinants can be classed according 

 to the outside markers present in the recombinant chromosomes, as is 

 done in Figure 10 for the crosses described in Figure 2. 



Let us now try to assign an origin to each type of recombinant 

 chromosome. If we regard the largest class in each cross, the single 



