148 STRUCTURAL HYBRIDS 



which were paired in part of their length at pachytene might be 

 unpaired at metaphase owing to failure of any chiasmata to be 

 formed. In the fragments of Secale as well as in the B chromosomes 

 of Zea (Rhoades and McClintock, 1935) there can be no doubt that 

 this is so (cf. Erlanson, 1933). 



(c) Pairing of large chromosomes with large and fragments with 

 fragments is commoner than cross-pairing, except in Lilium. Cross- 

 pairing never occurs in Solanum (tetrasomic in respect of the frag- 

 ment). In Matthiola, which is trisomic, the effect is still more 

 drastic, since the odd fragment, deprived in competition of a 

 partner amongst the large chromosomes, is evidently often left out 

 of association and fails to reach two-thirds of the expected frequency. 

 This indicates the importance of competition in pairing where there 

 is a choice of partner : those which have the longest sequence of 

 homologous particles will pair disproportionately often. This is 

 particularly important in considering " differential affinity " in 

 hybrid polyploids (Ch. VI) and (Enothera (Ch. IX) and pairing in 

 undefined hybrids. 



(iii) Translocation Heterozygotes. When a portion of one chromo- 

 some has been translocated to another the heterozygote shows 

 pairing of the corresponding segments. If one is terminal and the 

 other interstitial, the result is a lateral chiasma. This has been 

 observed in Tradescantia virginiana, Aucuba japonica and a Viola 

 hybrid (J. Clausen, 1931 c). The special behaviour found in the 

 hybrid Vicia saliva x V. angustifolia dolichosoma (Sveshnikova, 

 1929, a and b) can also be taken as evidence of translocation. An 

 open chain of four chromosomes is sometimes formed at meiosis, 

 but more usually one or more, sometimes all, of these appear as 

 univalents. The two chromosomes may be represented thus : 

 abode and/^ in one species, and abed and e/g in the other. It is to 

 be noted that failure of pairing is particularly frequent in the 

 shortest chromosome [cf. Fig. 54). 



(iv) Reduplication Heterozygotes. In certain trisomic individuals 

 of Datura, the so-called " secondary trisomies " (Belling and 

 Blakeslee, 1924 a), the extra chromosome was found to have the 

 same pairing properties at its two ends. It was therefore sup- 

 posed that this chromosome consisted of two identical segments 



