146 



STRUCTURAL HYBRIDS 



Avena sativa {41 +/) 

 Sorghum verticilliflorum (20 + 2 //) 

 (iii) Gymnosperms. 



Taxits canadensis (24 + /) 



Nishiyama, 1933. 

 Huskins and Smith, 1934. 



Dark, 1932. 



Table 17. 



Chiasma Frequencies of Fragments (calculated from metaphase 

 pairing on the chiasma theory of pairing, and compared with 

 those of major chromosomes, D., 1930 c ; Phiip and Huskins, 



1931). 



Species. 



Fritillariaimperialis 

 (i) "Yellow" . 

 (ii) " No. 10 " . 

 (iii) " Crown 



upon Crown" 

 Matthiola incana 

 (i) " Smooth " . 

 (ii) " Slender " . 



Number of 



Fragments 



(even or odd 



multiple). 



6ff. 



3ff(X2/3) 

 3ff(X2/3) 



I f. 

 I f. 



Chiasma-frequency 

 of Fragment. 



Calculated. Observed 



0-29 



0-37 

 0-31 



1-32 

 0-75 



0-22 



0-35 

 0-23 



078 

 0-36 



* This frequency has been calculated from the behaviour of the trivalent. 

 No correction for reduction of chiasma-frequency in trivalents need therefore 

 be made for the fragments. 



These sources of error are not quantitatively definite and the 

 observations lead to the following important conclusions : — 



(a) Fragments fail to pair, as predicted, and this failure is 

 evidently due to failure of chiasmata, since clones of Fritillaria 

 imperialis with low chiasma frequencies have a lower frequency 

 of fragment-pairing than those with a high chiasma frequency. 



(b) Pachytene association of small fragments is probably liable to 

 incompleteness, as expected, since the frequency is always below 

 the expectation on the assumption of completeness. The pairing 

 expectation is exactly fulfilled in Secale, where the fragments are 

 larger. The major chromosomes have 2-4 chiasmata per bivalent, 

 the fragments, one-third their length, o-8. Their pairing frequency 

 is slightly less owing to some fragments having two chiasmata 

 (Fig. 49). In triploid Hyacinthus it was inferred that chromosomes 



