214 Miscellaneous Linkage Topics 



rangement of pairs of chromosomes has been termed somatic 

 pairing. The exact cause of this somatic pairing has not been 

 determined but has been explained as resulting from exaggerated 

 forces of attraction. 



In the male Drosophila, during the first meiotic division, there 

 is no pairing at zygotene or chiasma formation at pachytene in 

 the autosomes and the peculiar pairing at metaphase has been 

 attributed by Darlington to the same type of forces responsible 

 for somatic pairing. Darlington, however, considers that the 

 autosomes are unique in this respect and that the sex chromo- 

 somes pair by chiasmata. However, chiasma pairing between 

 the X and Y chromosomes cannot be considered entirely normal 

 for this pairing apparently occurs only between their homologous 

 inert regions and because two reciprocal chiasmata must always 

 form. Since this reciprocal chiasma formation occurs only in 

 the inert region, it cannot be detected by genetic means, for the 

 interchanged segment bears no genes. There must always be 

 two chiasmata in this inert region and they must always be 

 reciprocal, as any other arrangement would produce new chro- 

 matids which could be detected genetically, and such new chro- 

 matids never form. Darlington's explanation for chromosome 

 pairing in such organisms, then, assumes two different mechan- 

 isms, one for the autosomes and one for the sex chromosomes. 

 It assumes, furthermore, that the sex chromosomes operate by 

 the usual meiotic mechanism but by a very restricted form of it 

 that has some strong elements of improbability. 



A theory for chromosome pairing which does not involve chias- 

 mata has been suggested by Cooper for the male of Olfersia 

 bisulcata, a parasitic fly from Panama, and it has also been 

 applied to Drosophila males. In the Olfersia male are three 

 pairs of autosomes. One type is large and V-shaped, one is 

 long and rod-shaped, and the third is small and dot-like. There 

 are also a large submedian chromosome and a smaller median 

 one which are believed to be the X and Y chromosomes. Since 

 this relationship has not been proved conclusively, they are 

 designated as X' and Y'. During the somatic mitoses that pre- 

 cede meiosis, the autosomes show decided somatic pairing at 

 midprophase, although they do not pair through their entire 

 lengths. The rod-shaped pair are associated only at their distal 

 regions whereas the V-shaped homologues are together at three 



