410 



BREAKDOWN OF GENETIC SYSTEMS 



some behaviour, and therefore capable of independent adaptation. 

 Such adaptation has led to the co-ordination which secures regular 

 reduction in normal meiosis as well as to the special systems referred 

 to above and discussed in Chapter XI. 



(iv) The Behaviour of Unpaired Chromosomes. Unpaired chromo- 

 somes have the same general type of behaviour whether in numerical, 

 structural or undefined hybrids or organisms with genotypically 



DIARINCSIS 



ME TA PHASE 



SPEftU 



SPERMATIDS \A 



Fig. 126. — A scheme of the spermatogenesis in normal males (above), 

 in B? X A^ hybrids (middle), and in A$ x Bi^ hybrids 

 (below) of Drosophila psendo-obscura. (Dobzhansky, 1934.) 



determined failure of pairing. One description will therefore 

 apply to all these cases. 



Unpaired chromosomes usually lie at random on the spindle at 

 metaphase. They do not move towards the equator as early as 

 the paired chromosomes. It is sometimes stated that unpaired 

 chromosomes lying to one side of the plate are moving to the pole 

 in advance of the bivalents at anaphase, but this conclusion is 

 unjustifiable. Their position is due to their never having reached 

 the plate, and they actually do not move until after the bivalents 

 have divided. 



