374 



PERMANENT HYBRIDS 



constitutes the pairing segment, it is clear that the mechanism of 

 chiasma formation, or whatever substitute for chiasma formation 

 is effective in pairing will not be able to secure the same regularity 

 in pairing as in the autosomes. Such is the case as shown both by 

 direct observation and inference from the occurrence in genetical 

 experiments of individuals with one or with three instead of with 

 two sex chromosomes. 



Where pairing fails, the unpaired chromosomes may divide at 

 the first division {e.g., in Fragaria), but will in any case be distributed 

 amongst the four daughter-cells at random with regard to one 



m^j 



Fig. ii8. — First metaphase in males of different races of Chrysopa 

 vulgaris. Y varies in size and segregates from X without 

 pairing {" distance-conjugation "). X 1600. (Naville and 

 de Beaumont, I933) 



another. Some will therefore have both the X and the Y 

 chromosome. This result has been referred to in Drosophila 

 (cf. Stern, 1929 a) as following non-disjunction, but is, as we now 

 see, often more properly to be ascribed to non-conjunction, i.e., 

 failure of pairing. 



True non-disjunction is, however, regularly found with multiple 

 chromosomes, especially the chains of three and four chromosomes 

 found in Humulus, Rumex and Phragmatohia. To give the normal 

 sex segregation, successive chromosomes must pass to opposite 

 poles, so that half the gametes have the X chromosome and half the 

 y chromosomes (Fig. 113). This segregation occurs in 85 per cent. 



