RECOMBINATION ANALYSIS IN MICROBL\L SYSTEMS 41 



separate. A meiotic pair thus consists of four closely associated strands 

 of hereditary- material, or chromatids, each of wliich will gi\e rise to a 

 cliromosome when the reduction di\isions are complete. The four 

 strands are seen to be tangled in places as they separate during meiotic 

 division, and breakage and fusion occur. If the broken ends of the 

 crossed strands fuse so tliat there is an exchange of segments, recipro- 

 cal crossing-o\er is produced. 



Today the whole problem of crossing-o\er is a completeh- open 

 one and has become one of the most exciting aspects of genetics. In- 

 deed, the genetic studies on microorganisms w hich lia\e gi\en birth to 

 the molecular theory of genetics ha\e reopened the subject of recom- 

 bination and infused it ^^"ith new ideas. Recombination has become a 

 problem in growth— the formation tlirough growth of chromosomes, or 

 even DNA molecules, which are hxbrid with respect to two ( or more ) 

 parental cliromosomes or parental DNA molecules. The present paper, 

 WTitten mainly for biologists with no special knowledge of genetics, 

 will be an account of one aspect of this rcNolution: nameh, die inter- 

 pretation of quantitati\e recombination data in terms of molecular 

 structure. 



Classical recombination theory 



Let us review briefl>- the quantitati\e rules that operate in classical 

 recombination studies. Genes that show a tendenc\- to be inherited to- 

 gether are located on a single chromosome and aie called linked genes. 

 A pair of linked genes ma\-, howe\er, become separated from each 

 otlier through crossing-over, as mentioned above (see Figure 1). The 

 frequency \\ith which linked genes recombine tlirough crossing-over is 



A + 



-O \ 1 



D 



A + -O \ ^- 



H 1 1 



-> 



-I- D -O \ i 



H \ 



+ 



-O \ ^- 



Figure 1. Reciprocal recombination through crossing-over between two non- 

 sister chromosome strands at meiosis. The dotted line indicates one of tlie 

 possible positions of the cross-over which would \-ield die fonr chromosomes 

 at die right after di\'ision of the centromere. 



