Section 2 — Recombination 



nineteen alleles at the me-2 locus were classified 

 with respect to the markers present on both sides 

 of the me-2 gene. One of the two classes of 

 methionine prototrophs having markers re- 

 combined occurred in excess over the other; 

 when the markers entered the cross in the op- 

 posite phase, a similar excess was found in the 

 reciprocal class. The linear order of me-2 alleles 

 determined from these asymmetries was consis- 

 tent with a map based on prototroph frequencies. 



Pronounced asymmetries were also observed 

 in the numbers of the two parentally-marked 

 classes of prototrophs; when the markers en- 

 tered the cross in the opposite phase these asym- 

 metries were reversed. The asymmetries between 

 the two parentally-marked classes were correlated 

 in direction with the asymmetries between the 

 two classes having markers recombined. 



The results may be interpreted in terms of 

 multiple exchanges within small, discontinuously 

 distributed, regions of effective pairing — the 

 asymmetries would result from a reduction of 

 coincident exchanges in the region proximal to 

 the selected interval. 



The coincident exchange frequency in the 

 proximal region is influenced by the position of 

 the more proximal of the two me-2 alleles, but is 

 independent of the recombination frequency 

 between the two alleles. The reduced coincidence 

 in the proximal region could be explained if the 

 me-2 region is situated immediately distal to 

 some discontinuity which imposes a nonran- 

 domness of exchanges by reducing the chance 

 that the effectively paired region could be proxi- 

 mal to, yet extend into, the me-2 region. 



Supported by Public Health Service research 

 grant El 462. 



2.7. Conversion and Crossing-over as Recombination 

 Mechanisms in Ascobolus immersus. 

 W. Gajewski, A. Kruszewska, A. Makare- 

 wicz, A. Paszewski, S. Surzycki and H. Bie- 

 lawska (Warsaw, Poland). 



Crosses between mutants with white ascos- 

 pores from one series give wild type, dark re- 

 combinants due predominantly to conversion. 

 The frequencies of conversion-type asci are 

 roughly proportional to the distances between 

 the sites involved which enables one to map them. 

 When mutants from one series are crossed with 

 wild type strain the frequencies of 2:0 asci 

 increase in percentage from one end of the series 

 to the other approaching the value of recombina- 

 tion between the extreme sites of the series. 



This is interpreted that the frequency of 6:2 

 asci in crosses between two white mutants 

 represents only that part of copy choices which 

 begin or end in between two sites. It seems that 

 the beginning of the switches is random along 

 the analysed unit whereas the return to the origi- 

 nal matrix is rather nonrandom. 



One mutant, namely 186, crossed with wild 

 strain shows very high frequencies (10- 12 per cent) 

 of 6:2 and 2:6 asci of conversion type. Besides, 

 less frequent asci 5 : 3, 3 : 5, 7 : 1 andl : 7 also appear. 

 They are not due to aberrant chromosome 

 segregation or mutation. The asci 5:3 and 

 3 : 5 result probably from copy errors of half- 

 chromatids. The 7:1 and 1:7 asci may be the 

 result of two copy errors it two successive re- 

 plications. The tetrad analysis of visible asco- 

 spore mutants in Ascobolus immersus on suffi- 

 ciently large scale reveals the predominant role 

 of conversion as a mechanism of recombina- 

 tion within a small piece of genetic material . 



2.8. Effect of Crossing-over on the Process of Sponta- 

 neous Mutation. G. E. Magni (Pavia, Italy). 



It has been recently observed that the rates of 

 spontaneous mutations occurring during meiotic 

 division are higher than those occurring during 

 vegetative reproduction (mitosis) in the same 

 type of cells (yeast). W 



Further experiments have shown that most 

 spontaneous back-mutations at locus hii-i 

 during meiosis, in either one of the two identical 

 alleles carried by diploid yeast cells, are con- 

 nected with crossing-over in the specific locus. 



Data regarding the amount of negative inter- 

 ference in the adjacent chromosome regions and 

 the polarization of the intragenic crossing-over 

 will be presented. 



The hypothesis of unequal crossing-over 

 within the locus leading to back mutant chroma- 

 tids will be discussed. 



1. Magni and Von Borstel, Genetics 47, 1097, 

 1962. 



2.9. Further Investigations on Somatic Gene Conver- 

 sion in | the Tomato. Rudolf Hagemann 

 (Gatersleben, Germany). 



In heterozygous sulf+sulf plants a green 

 yellow variegation occurs (sulf means any mutan 

 allele of the chlorophyll deficient sulfurea series 

 either of the sulp UT3 - or of the sulf™« group) 



11 



