CHAPTER V 



STRUCTURAL HYBRIDS 



Definition and Classification of Hybrids — Structural Hybrids — Fragments— 

 Translocation and Interchange Heterozygotes — Unequal Chromosome Pairs. 



So wenig man eine scharfe Unterscheidungslinie zwischen Species und 

 Varietaten zu ziehen vermag, ebenso wenig ist es bis jetzt gelungen einen 

 griindlichen Unterschied zwischen den Hybriden der Species und Varietaten 

 aufzustellen. 



Mendel, 1866. 



I. CLASSIFICATION BY FUNCTION 



The systematist recognises a hybrid as an individual or type 

 intermediate between two species, and he assumes it to be either a 

 first cross or a derivative of a first cross. The geneticist's definition 

 of a hybrid, which he owes to Mendel, is entirely different. On the 

 one hand, he cannot draw a sharp line between species-differences 

 and varietal differences, because he finds they are of the same 

 kinds and differ only in degree. Even in regard to inter-sterility, 

 every gradation occurs between those pairs of species which are 

 prevented from crossing in nature merely by incidental conditions 

 and those which may be assumed to be intersterile under any 

 conditions. For the geneticist, therefore, any cross between dis- 

 similar races must be a hybrid, however slight their differences. 

 On the other hand, the geneticist makes a sharp distinction between 

 the first cross [F-^) and its derivatives, because the first cross is 

 capable of showing segregation of the parental differences in its 

 offspring, while its derivatives need no longer have these differences. 

 An intermediate character is therefore for him no criterion of 

 hybridity. 



And finally the geneticist is not concerned with the difference 

 between the parental zygotes but with that between the gametes 

 which actually fused to make the hybrid or which are formed at 

 meiosis in the hybrid. We define hybridity by function and not by 



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