368 



PERMANENT HYBRIDS 



Or in other words we may say that the pairing segments may be 

 reduced to a minimum, while the differential segments, although 

 genetically different, remain the same size in X and Y. 



The pachytene observations seem to conflict with the assumption 

 of differential and pairing segments, for in Enchenopa X and Y are 

 paired throughout their length and in Humidus and Rumex a 

 complete triradial association is often found, the two short arms of 

 the Y's which never form chiasmata being associated (Kihara and 

 Hirayoshi, 1932). Clearly in these cases we are dealing with non- 

 homologous torsion pairing, which as we have seen earlier is probably 

 characteristic of all structural hybrids, but which implies neither 

 homology as a cause nor crossing-over as a consequence. 



Where the differential segment is proximal, i.e., where it 

 includes the centromere, the simplest kind of metaphase pairing 

 and segregation results. This is the condition found in all plants 

 and most animals. It has usually been said in describing this type 



2 Occasionally failure of pairing, or non-disjunction resulting from it, 

 observed at meiosis. 



^ Variation found in Y chromosome within the species. 



* Second division reduction. 



« The evidence for Y is genetical. 



