Section 7 — Cytogenetic:, 



or progeny of the new trisomic plants. Some 

 problems in the linkage studies using trisomies 

 are considered in connection with the occurrence 

 of such pericentric inversions accompanied by a 

 deletion as has been reported here in this paper. 



7.16. Genetic Recombination and Chromosome 

 Disjunction in a Balanced Tertiary Trisomic of 

 Barley. R. T. Ramage (Tucson, U.S.A.). 



The interchanged chromosome 2"' A is broken 

 close to the centromere in the short arm of 

 chromosome 2. The MS2UIS2 locus is tightly 

 linked with the breakpoint of 2 7 ( and is carried 

 on chromosome 2J (arm not known). The Vv 

 locus is carried on the long arm of chromosome 

 2 and, in normal stocks, exhibits about 26 per 

 cent recombination with the Ms2ms2 locus. A 

 balanced tertiary trisomic was set up so that the 

 two normal chromosomes carried the v and ms2 

 alleles and the interchanged chromosome 2\ 

 carried the V and Ms2 alleles. In the balanced 

 tertiary trisomic the frequency of recombination 

 between the genes V and ms2 was measured by 

 determining the frequency of male gametes 

 carrying the V allele. Chromosome disjunction 

 types, which are dependent upon the type of 

 pachytene pairing, were obtained by determining 

 the frequencies of microspore abortion and of 

 primary trisomies in the progeny of the balanced 

 tertiary trisomic. Microspore abortion and the 

 production of primary trisomies are results of 

 the same pachytene pairing event and should be 

 found with the same frequency. If genetic re- 

 combination is dependent upon an exchange of 

 chromosome parts at pachytene, the frequency 

 of microspore abortion, or of the production of 

 primary trisomies, should equal one-half the 

 frequency of male gamete carrying the V allele. 

 In these studies, much more genetic recom- 

 bination was observed than can be accounted 

 for by an exchange of chromosome parts at 

 pachytene. 



7.17. Chromosome Segregation in a Tertiary Trisome 

 of Secale cereale in its Three Karyological 

 Backgrounds. J. Sybenga (Wageningen, The 

 Netherlands). 



Tertiary trisomes originate from reciprocal 

 translocation heterozygotes. The extra chromo- 

 some can occur in three "backgrounds": the 

 translocation homozygote, the translocation 

 heterozygote and the normal type. All three 

 forms may be found in the progeny of a selfed 



translocation heterozygote, that is trisomic for a 

 translocation chromosome. 



Meiotic configurations depend on the "back- 

 ground", and chromosome segregation depends 

 on these configurations. In the translocation 

 homozygote background the trisome is in fact a 

 primary trisome. In the normal background as 

 largest configuration a chain of five can be for- 

 med. In the heterozygous background this is 

 again a chain of five that, with zigzag orientation, 

 segregates into the translocation complement 

 with the extra chromosome against the normal 

 complementwithout it, or the reverse. With equally 

 complete pairinga bivalent and a chain of three 

 may be formed, segregating into the translocation 

 complement without the extra chromosome 

 against the normal complement with it. The third 

 possibility is a ring of four with a univalent, that 

 leads to loss of the extra chromosome, or inclu- 

 sion in either group with equal probability. Less 

 complete pairing or non-zigzag orientation 

 leads to increased loss of the extra chromosome, 

 increased randomness of its distribution and 

 increased sterility. 



In the homozygous backgrounds no segrega- 

 tion in regard to the translocation occurs. The 

 type of configuration affects the frequeny of the 

 extra chromosome in the progeny and the 

 sterility. Meiosis and chromosome segregation 

 were studied in one tertiary trisome in all three 

 backgrounds in Secale cereale. Orientation of 

 the multivalents was predominantly zigzag. 

 Preference for type of segregation in the hetero- 

 zygote could not be demonstrated. Apparent- 

 ly the translocation did not tend to fa- 

 vour pairing between completely homologous 

 chromosomes only. This agrees with the results of 

 Ahloowalia on the autotetraploid translocation 

 heterozygote. Data on meiosis and chromosome 

 segregations will be presented. 



7.18. Transmission of Rye Chromosomes in Mono- 

 somic Addition Lines of Rye Chromosomes in 

 Wheat. Shams-ul-Islam Khan (Lahore, 

 Pakistan). 



In the monosomic addition lines of rye 

 (Secale cereale L.; "Swat Rye", In = 14) 

 chromosomes in wheat (Triticum aestivum L. ; 

 C271, 2« = 42), transmission of individual rye 

 chromosomes has been studied, both through 

 male and female gametes. Ratios (42/43) 

 through male and female gametes are: 

 I (4.78 : 1/5.0 : 1), II (2.8 : 1/3.0 : 1), III 

 (2.0 : 1/2.25 : 1), IV (12.0 : 1/10.0 : 1), V 

 (3.0 : 1/2.9 : 1), VI (8.0 : 1/6.0 : 1), VII 



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