SECTION 2 



RECOMBINATION 



2.1. Factors Affecting Distributive Pairing between 

 Nonhomologues in Drosophila melanogaster. 

 Rhoda F. Grell (Oak Ridge, U.S.A.). 



Meiotic associations that lead to the regular 

 separation of nonhomologous chromosomes 

 occur with high frequencies in Drosophila me- 

 lanogaster females of particular genotypes. 

 The relation of the size of the nonhomologues 

 to the regularity of their association has been 

 investigated by measuring the nondisjunction 

 frequencies between a free fourth chromosome 

 and a series of X-duplications in the progeny of 

 females of the genotype y 2 /j> 2 /Dp(l ;f),y + ; T(3; 4) 

 86D, m//T(3 ; 4)86D, red; ci D mated to xy , 

 yB/Y males. The duplications were obtained and 

 characterized by Cooper and Krivshenko and 

 vary in size from ~ 0.3. to 3.0 times the meta- 

 phase length of chromosome four. The percent of 

 nondisjunction between chromosome four and 

 the duplication, given in ascending order of 

 duplication size, are, / ~ 0.3 = 4.49 ± 0.19, / 

 0.5 = 0.39 ± 0.07, / 0.7 = 0.12 ± 0.06, 0.9 = 

 0.08 ± 0.03, 1.0 = 0.13 ± 0.05, 1.1 = 0.02 ± 

 0.02, 1.1 = 0.08 ± 0.03/ 1.4 = 0.38 ± 0.08, 

 1.6 = 0.39 ± 0.08, 2.0 = 0.42 ± 0.07, / 

 2.5 = 2.03 ± 0.39, 3.0 = 1.89 ± 0.24 /. The 

 lowest frequencies of nondisjunction (~ 0.08 per 

 cent) occur with those duplications closest in size 

 to chromosome four (0.7 - 1.1) whereas the 

 highest frequencies occur with the smallest 

 (~ 0.3) and largest (2.5 and 3.0) duplications. 

 The results indicate size alone is not responsi- 

 ble for pairing efficiency since the progressive 

 change in duplication size is not associated 

 with correlated changes in nondisjunction fre- 

 quency. Instead, the duplications fall into five 

 discrete classes (designated by slashes in the 

 sequence above) within which size variation 

 causes no apparent related change in segregation 

 behavior. This suggests the possibility of sites 

 that participate in nonhomologous associations 

 as has been postulated by Gershenson and 

 Cooper for homologous associations. 



2.2. Nonrandom Assortment of Compound Chromo- 

 somes with Nonhomologous chromosomes in 

 Oocytes of Drosophila melanogaster. E. H. 

 Grell (Oak Ridge, U.S.A.). 



The requirements for nonrandom assoi cment 

 of nonhomologous chromosomes was studied 

 by R. F. Grell (1962). For noncompound 

 chromosomes it was determined that a chromo- 

 some must not have been a crossover. (The 

 term compound chromosome is intended to 

 include chromosomes formed by the joining of 

 two homologous chromosome arms to one 

 centromere.) If two nonhomologous chromo- 

 somes are noncrossovers they may pair and pass to 

 opposite poles of the first meiotic division spindle. 

 The pairing in which nonhomogolous elements 

 may participate has been termed distributive 

 pairing to distinguish it from exchange pairing 

 which occurs prior to crossing over and only 

 involves homologous chromosomal regions. 



The studies reported here deal with distributive 

 pairing of nonhomologous chromosomes when 

 at least one member of the pair is a compound 

 chromosome. A familiar compound chromosome 

 is the attached X. The two arms are attached in 

 reverse order on either side of a centromere. 

 Crossing over occurs between the two arms 

 with almost the same frequency as for normal 

 free X chromosomes. Yet, from females of the 

 genotype xx /0; SMI, Cy/T(2;3)A, 90 per cent 

 of the recoverable gametes have the attached 

 X or SMI, Cy and only 10 per cent have both or 

 neither chromosome. Despite the fact that more 

 than 95 per cent of the tetrads contain an ex- 

 change, the attached X shows a very highly 

 nonrandom assortment with a second chromo- 

 some. Experiments that compare compound X's 

 with an exchange and those without an exchange 

 indicate that in contrast to free X's, exchange 

 has no influence on the extent of nonhomologous 

 pairing. All compound chromosomes tested 

 (various compound X's and the attached 4's) 

 have shown nonrandom assortment with non- 

 homologous chromosomes. 



2.3. On the Nature of the Event that leads to Recom- 

 bination of Pseudoalleles of the m-dy locus in 

 Drosophila. Allan B. Burdick, Robert A. 

 Shleser, Evelyn Barbour Bendbow, and 

 Rosanne Abbadessa (Lafayette, U.S.A.). 



The recombination map of the m-dy locus 



