Section 7 — Cytogenetics 



carry the translocation. After X-irradiation, 

 twenty-seven cases of inherited partial sterility 

 were found in Habrobracon. Of these, twelve of 

 the females had hatchabilities between 30 and 

 40 per cent. Six were below 30 per cent, four 

 were above 60 per cent, and five were between 

 40 and 55 per cent. By classic theory, it can be 

 assumed that the 40-55 per cent class involved 

 adjacent I segregation and the 30-40 per cent 

 class involved mostly adjacent I and adjacent II 

 segregation. The other classes require additional 

 interpretations such as multiple chromosome in- 

 volvements for those with hatchabilities lower than 

 30 per cent, and translocations involvingchromo- 

 some ends for those with hatchabilities above 60 

 per cent. The time of death of the duplication- 

 deficiency products is extremely regular among all 

 of the induced cases, occurring at midem- 

 bryonic stages of development. On the other 

 hand, embryonic recessive lethal mutations are 

 expressed at all stages of embryonic develop- 

 ment from the blast ula stage to hatching. Very 

 few cases of inherited partial sterility are found 

 in progeny from females irradiated in prophase I 

 or metaphase I; inherited partial sterility is com- 

 monly induced after irradiation of males with 

 mature sperm. 



7.24. Crossing-over in the Sex Bivalent of Male 

 Mammals. J. Wahrman and U. Ritte (Jeru- 

 salem, Israel). 



A knowledge of the pairing relationships 

 between the X and Y chromosomes of mammals 

 and man is essential for an understanding of the 

 heredity of X-borne genes, and especially of 

 partial sex-linkage. 



In the meiosis of Apodemus mystacinus, a 

 murine species of Israel, pre- and post reductions 

 occur normally side by side. Chiasmata can be 

 directly demonstrated at first meiotic prophase. 

 The occurrence of two disjunctional types may 

 be observed at first anaphase and is confirmed 

 by the inspection of secondary spermatocytes. 

 Some of these contain X-dyads or Y-dyads 

 resulting from pre-reduction, while others clearly 

 exhibit the heteromorphic X-Y dyad awaiting 

 post-reduction. 



In Apodemus sylvaticus the sex bivalent is less 

 favourable for analysis but a large proportion 

 of secondary spermatocytes displays the X-Y 

 dyad, which suggests the previous existence of a 

 chiasma. Thus the examination of secondary 

 spermatocytes may furnish critical evidence for 

 the existence of chiasmata in species where these 

 cannot be directly observed. 



In certain mammalian species a small fraction 



of primary spermatocytes exhibit a chiasma 

 between X and Y, although none can be seen in 

 the majority of cells. Finally, in most mammalian 

 species, including man, the X and Y appear in 

 end-to-end configuration, which is generally 

 attributed to non-chiasmic association. This 

 point requires re-examination. 



The occurrence of chiasmata in the male sex- 

 bivalent, and therefore probably of genetic re- 

 combination, is firmly established in at least 

 certain mammalian species. 



7.25. Interchromosomal Effects on Crossing-over in 

 Drosophila melanogaster. David T. Suzuki 

 (Edmonton, Canada). 



Crossing-over was measured in chromosome 3 

 of females heterozygous and homozygous for 

 the X-chromosome inversions v 3p , j> 4 , sc 4 , sc 8 , 

 sc 9 , dl-49, w m4 , and rst 3 . Crossover frequencies 

 were increased in all inversion heterozygotes and 

 in all but the sc 9 and dl-49 homozygotes. The 

 lack of effect in sc 9 and dl-49 homozygotes in- 

 dicates that the interchromosomal effects of in- 

 version heterozygotes are not due to the intrinsic 

 properties of the rearrangement per se. Since the 

 inversions which had effects when homozygous 

 have the X-tip apposed to heterochromatin (y 4 ) 

 or heterochromatin located distally, it was 

 suggested that the position of heterochromatin 

 adjacent to the tip region was responsible for the 

 interchromosomal effects of the inversions. 

 Fragment-1 (Y S X - ), a normal X chromosome 

 with the short arm of the Y chromosome attached 

 to its tip, was tested and crossing-over was shown 

 to be significantly increased in chromosome 3 of 

 Fragment-1 homozygotes and heterozygotes. 

 Since crossing-over was not changed in females 

 homozygous and heterozygous for a normal X 

 with Y-short attached to its centromere (X'Y S ), 

 the distal position of the Y heterochromatin 

 appears to be important for the interchromoso- 

 mal effects of a Y-arm attachment. 



7.26. The Mode of Interchromosomal Effect of In- 

 version Heterozygosity on Crossing-over Fre- 

 quency in Drosophila melanogaster. Tarvo 

 Oksala (Turku, Finland). 



The mechanism by which inversion hetero- 

 zygosity affects crossing over in heterologous 

 chromosomes is still unknown. In order to eluci- 

 date this problem the author has carried out a 

 few experiments in which the (structurally nor- 



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