420 BREAKDOWN OF GENETIC SYSTEMS 



say how many or of what kinds these may be. Moreover, their 

 occurrence is so rare that we can know nothing accurate about their 

 frequency. The best indication we can get is from mitosis in 

 exceptional organisms with a genotypically controlled high frequency 

 of structural change. 



Two stocks have been discovered with frequent structural 

 changes. The more extreme was the one derived from a Tnticum- 

 Secale hybrid (Plotnikova, 1932) which showed as many as one- 

 fifth of the mitoses with chromatid bridges and fragments. These 

 are clearly the result of the formation of dicentric and acentric 

 chromosomes by asymmetrical interchange, Hke those formed by 

 crossing-over in dyscentric hybrids at meiosis. This type of 

 interchange, let us remember, is precisely the type that will show 

 itself at mitosis as no other will. And it is also precisely the type 

 which will be eliminated during somatic life and, will consequently 

 never appear at meiosis or in the progeny (Plate XII). 



A less extreme frequency was found by Beadle in the " sticky 

 chromosome " mutant of Zea Mays (1932). A few changes in size 

 and number at mitosis indicate that structural change is occurring 

 with unusual frequency. At meiosis the result is a great but 

 variable confusion in the association of the chromosomes. Some- 

 times a single chromosome group is formed as though a dozen 

 translocations had taken place. At anaphase several bridges are 

 found, showing that the changes include inversions. Beadle 

 attributes this behaviour to " stickiness " of the chromosomes, but 

 the spindle is normal and regular tetrads can be formed. The 

 abnormality seems therefore to be sufficiently accounted for by a 

 high rate of structural change. 



If the natural change-rate is once in a million mitoses, the Zea 

 rate is perhaps once in a thousand mitoses, and the Triticum-Secale 

 rate once in a hundred. With X-ray treatment, as we shall see 

 later, the rate may be increased to ten times in one mitosis ; the 

 limit is merely the viability of the cell, of the tissue, or of the 

 organism. 



Normal changes continue or may even be hastened in resting 

 seed, according to the work of Navashin (1933) on Crepis tectorum 

 raised from six-year-old seed. As in the " sticky " plants in Zea 



