with different regions of the Y chromosome in meiosis to give a trivalent 

 XiYXo in the male and two bivalents XiXi XoXo in tlie female. In a 

 number of dioecious species of plants such as Spinacia oleracea with 

 presumed XY systems, there is too little morphological difference be- 

 tween the two elements to allow cytological distinction. In such cases it 

 is assumed that the two elements differ with respect to a limited segment 

 in which crossing over is completely, or at least, largely suppressed. In 

 some cases. Cannabis saliva, for example, the X and Y elements are so 

 similar in size and shape that the heteromorphic character can be dem- 

 onstrated only with difficulty by careful measurement of the two ele- 

 ments. In other cases, as in several species of the marine worm Bonellia, 

 bisexuality appears to be largely the result of environmental influences 

 during larval development; and in another marine worm, DimopJiilus, 

 the female lays two kinds of eggs, small ones which become males and 

 large ones which become females. Since the size difference is estab- 

 lished before meiosis, no chromosome mechanism would seem to be 

 involved. 



In certain animal groups, especially the Hymenoptera (bees, wasps, 

 etc.), males are haploid derived from unfertilized eggs and females 

 diploid derived from fertilized eggs. Cytologically, therefore, the differ- 

 ence between sexes is ascribable to a difference of an entire chromosome 

 complement. What this difference means in genetic terms is not entirely 

 evident, but from the extensive studies of the Whitings (1943, 1945) 

 on Habrobracon, femaleness se^ms attributable to heterozygosity of a 

 number of loci since inbreeding leads to production of some rather 

 inviable but otherwise normal diploid males. 



CYTOGENETICS AND THE FINE STRUCTURE OF CELLS 



In classical genetics, the major problem was the mechanism by which 

 genetic potential was transferred from parent to offspring, so that cyto- 

 genetics was concerned with cellular changes, notably in the chromo- 

 somes, which influenced such transmission. In recent years geneticists 

 have become increasingly concerned with the problem of translation of 

 genetic information into specific action. By extension, a valid concern 

 of cytogenetics might well be with changes in fine structure which may 

 have a bearing on the transfer of genetic information from the chromo- 

 some to sites of action in the cytoplasm in either a general or specific 

 sense. In particular one may attempt to correlate changes in fine struc- 

 ture of chromosomes with genetic activity in a fashion similar to that 

 adopted by Beermann (1956, 1959) in attempting to relate "puffing" 



190 / CHAPTER 8 



