i68 STRUCTURAL HYBRIDS 



tion that only identical genes may pair, the first will have a 

 negligible effect as compared with the second. Therefore the 

 assumption must be made that all changes effective in reducing 

 pairing, apart from changes of number, are intergenic or 

 structural. 



Structural changes need not in themselves mean any change of 

 genetical properties in the organism, except in a mechanical sense, 

 since the same materials may be arranged in a different way without 

 any phenotypic effect. But structural changes will nearly always 

 lead to changes in proportion through recombination, and these 

 have an important genetic significance. Their occurrence is 

 therefore a measure of differentiation. 



(b) Since organisms vary (within and between species) in the 

 frequency and distribution of chiasmata in each unit of the length 

 of paired chromosome, it follows that the pairing in different 

 hybrids between species and between varieties of the same species 

 cannot be compared as an indication of similarity without a 

 knowledge of chiasma frequencies in the parents. 



We are therefore led to expect that, in some hybrids between 

 species, chromosomes will pair, in others they will fail to pair, and, 

 in others again, their behaviour will be intermediate and will then 

 vary in accordance with the variation in chiasma formation. The 

 differences of behaviour between hybrids will have a very indirect 

 relation to the differentiation in genetic properties of the parent 

 species. 



4. CHROMOSOME PAIRING IN UNDEFINED HYBRIDS 



Undefined hybrids in which parental gametes have the same 

 chromosome number, or a slightly different one, for no definable 

 reason, are of three kinds : (i) those with little or no pairing at 

 metaphase ; (ii) those with partial, and, always, variable pairing ; 

 (iii) those with complete or almost complete pairing (v. Table 21). 

 Prophase conditions have not been suitable for exact study in most 

 of these hybrids, but failure of metaphase pairing has frequently 

 been found to follow incomplete pachytene pairing, as would be 

 expected (cf. Cretschmar, 1928). In other cases pairing appears to 

 be complete at pachytene (Federley, 1915), but it may well be 



