STRUCTURAL HYBRIDITY 



heterozygous for the new change. The simplest type is the inter- 

 change hybrid. Its movements resemble those of a tetraploid (or 

 tetrasomic) although their genetic consequences are quite different. 

 Interchange, by recombining segments, produces from two old 

 chromosomes, AB and CD, two new ones, BC and DA. When the 

 new and old types pair at pachytene in the hybrid they form a cross 

 arrangement like that given, with one exchange of partners, in a 

 tetraploid; but the position of the exchange of partner is fixed 

 at, or near, the position of the interchange. There is no choice 

 (Fig. 29). 



PACHYTENE PAIRING IN STRUCTURAL HYBRIDS 



0< 



a a 



<'< 



b b 



ccr 



DEFICIENCY DELETION 

 (Terminal) (Intercalary) 



INVERSION 



INVERSION 

 (or INTERNAL 

 EXCHANGE) 



INTERCHANGE 



Fig. 30. — The changes produced by loss, inversion and interchange illustrated by 

 their effects at pachytene in the hybrid. 



From the exchange system of pairing a ring of four, chain of 

 four, two chains of two, or a chain of three with a univalent, can 

 arise at metaphase. Again, as in the tetraploid, these different con- 

 figurations arise according to the chance distributions of chiasmata 

 and with frequencies depending on the lengths of the four pairing 

 segments. Hence we have some, but not all, of the metaphase figures 

 found in the tetraploid. What is more important, however, is that 

 the results of segregation are balanced only in one particular com- 

 bination, namely where AB and CD pass together to one pole, 

 and BC and DA to the other. In a flowering plant such as Datura, 

 therefore, whose spores have an independent life, AB + BC or 

 CD-\-DA, and BC + CD or AB-^DA combinations die 

 (Fig. 29). In Drosopliila, on the other hand, where the genetic con- 

 stitution does not affect the capacity of eggs or sperm, balanced 



Elements nf Genetics 1 29 I 



