RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 53 



one event increases the probability of the second, and less than one if 

 the occurrence of the first decreases the probability of the second. The 

 coincidence values of Cross 3 range from four to about 30 in the dif- 

 ferent experiments of Chase and Doermann, proving that the occur- 

 rence of a first cross-over enormously increases the probabihty of a sec- 

 ond with a neighboring segment of the chromosome. 



Thus recombination appears to obey two diflFerent laws, depend- 

 ing upon the distances between markers. In other words, the recom- 

 bination yardstick is not constant: its units shrink when we try to 

 measure long distances and expand when we try to measure short ones. 

 Therefore, when diflFerent orders of distance are considered, diametri- 

 cally opposed behavior is observed v/ith respect to the influence of one 

 cross-over event upon a second. Interference occurs when distances 

 are measured in terms of unitary values of recombination, but when 

 small distances are under study, not only is there no interference, but 

 there is a high incidence of multiple exchanges. 



The theory of effective genetic pairing 



Clearly, the theory that recombination frequency is a linear func- 

 tion of the distance between two points on the chromosome is unten- 

 able, at least without substantial modification. A simple modification of 

 the classical theory has been proposed by Rothf els ( 1952 ) and has re- 

 ceived development and substantial support from the work of Ponte- 

 corvo (1958) and Pritchard (1955, 1959). This consists of supposing 

 that pairing in the genetic sense is not equivalent to cytologically ob- 

 servable pairing; that genetic pairing precedes cytological pairing and, 

 far from being complete, involves only small regions of homologous 

 chromosomes. It is supposed to occur as a random encounter of cor- 

 responding regions of homologous chromosomes, prior to or during 

 chromosome duplication. According to the theory of partial genetic 

 pairing, crossing-over is a phenomenon of the very growth process that 

 forms a new chromosome; it is not a mechanical accident. 



The theory of partial pairing states that two distinct events are in- 

 volved in crossing-over, and with these two variables a complete ac- 

 count can be made of the genetic phenomena observed. There is on the 

 one hand the probability of an eflFective pairing occurring in a particu- 

 lar chromosome interval. This probability would be determined by the 

 length of the interval, for one thing, because a long interval would pro- 

 vide more sites for pairing than a short one. The probability could also 

 be determined in part by the degree of similarity of the homologues in- 

 volved. Further, when one region of a chromosome becomes eflFectively 

 paired, this may give rise to interference, since the paired chromosomes 

 are no longer free to pair in a second region. There is, on the other 

 hand, the actual recombination process, which will take place within 



