Section 2 — Recombination 



shows an unambiguous sequence of five se- 

 parable mutant elements extending over about 

 0.07 map units. The sequence is v . . . m 61e , 

 «7 59a , m, dy, dy eok . . . g with several other ele- 

 ments being inseparable from one or more 

 members of the sequence. A number of insertio- 

 nal-type crossover products (double and triple 

 crossovers with respect to the outside markers) 

 have been obtained from free-X recombina- 

 tion tests. These have involved nine different 

 elements of the locus, and their explanation 

 poses a problem as to the nature of the event that 

 leads to recombination of the elements of the 

 locus. Although the cis-trans test of the most 

 distal elements (m 61e and dy 60k ) indicates that 

 they occupy separate functional sites, comple- 

 mentation tests among the various elements 

 indicate that they could all belong to the same 

 cistron. The m 6le dy 60k m-phase double mutant 

 has been tested in //a/;s-phase with all of the 

 internal elements of the locus and has never 

 yielded a wild-type recombinant — that is, no 

 double crossing over takes place within the 

 locus — although m 61e dy 60k has broken down 

 several times to yield dy 60k types. Attached-X 

 recombinations of m 61e and m yielded 31 events 

 of which 14 were reciprocal. One of the m 61e 

 m m-phase double mutants has been /ra/w-phase 

 tested to m 61e , /?/ 59a , and m and has not given 

 any wild-type recombinants. Our present opinion 

 is that recombinations within this locus are of 

 the same classical reciprocal kind that have 

 been postulated for interlocus recombinations, 

 and that the insertional-type crossovers we have 

 obtained can be explained on a classical basis. 



Supported by a grant from the National 

 Science Foundation. 



2.4. Genetic Ultrafine Structure of the T4rII Region. 

 Irwin Tessman (Lafayette, U.S.A.). 



Recombination frequencies as low as 10 6 

 per cent have been measured in the rll region of 

 phage T4. At this sensitivity, hot spots in the 

 rll region have been resolved into two or more 

 genetic sites, separable by recombination. No 

 lower limit to recombination frequencies is 

 apparent. Therefore, lack of recombination 

 cannot guarantee identity of genetic sites. 



The largest recombination frequencies in the 

 rll region are greater than 1 per cent and the 

 lowest less than 10 -6 per cent. The ratio, 10 6 , 

 is greater than the number of nucleotides in the 

 entire phage. Therefore, in this range there 



must be extreme deviations from even rough 

 additivity of recombination frequencies. Within 

 the range of very low recombination frequencies, 

 marked deviations from additivity are, in fact 

 found. 



2.5. Effect of Negative Interference on Genetic Map 

 Concepts and Its Consequences for Intra- 

 allelic Recombination Studies. W. D. Hanson 

 (Raleigh, U.S.A.). 



Experimental studies involving intra-allelic 

 recombination have yielded results which are not 

 entirely compatible with classical recombination 

 concepts. Negative interference noted in intra- 

 allelic recombination studies suggested that 

 given a genetic recombination at a point one 

 could expect additional points of recombina- 

 tion or a "cluster" of points of recombinations 

 (one or more) within some interval. The products 

 of meiotic division were described in a proba- 

 bility space so that the probability of chromo- 

 some types could be formulated. The products 

 of meiosis for linked loci were formulated for 

 classical recombination studies and for intra- 

 allelic recombination studies assuming that 

 cluster recombinations could occur in some inter- 

 val. The results involved the probability of an 

 "odd cluster", length of cluster interval, and 

 genetic map distance between linked loci. Cluster 

 recombinations within a relatively short interval 

 should not upset classical recombination meas- 

 ures. The frequency of chromosome types 

 (with respect to marker genes) with selection for 

 intra-allelic recombinations reflected a bias 

 identified with cluster effects. Relative reverse 

 mutation rates could also be a factor; however, 

 for crosses involving a locus bounded by two 

 markers a contradiction existed for linear order 

 of alleles vs. relative mutation rates. A positive 

 bias in map distance involving a non-selected 

 marker was associated with cluster recombina- 

 tions which was a maximum for a closely linked 

 marker and decreased with increased map dis- 

 tance. Published data supported cluster recom- 

 bination concepts, as compared to mutation, 

 with a positive bias in recombination being a 

 consistent feature in these data. 



2.6. Polarized Intragenic Recombination in Neuro- 

 spora crassa. Noreen E. Murray (Stanford, 

 U.S.A.). 



Methionine-independent progeny from crosses 

 between many pairs of combinations involving 



10 



