428 Aneuploids and Nondisjunction 



gous for a lethal gene or deficiency and therefore do not survive. 

 The velans • gaudens type, however, carries two lethals, but they 

 are not the same. This type is not homozygous for either lethal, 

 it does not die in an early stage, and it produces Oe. La- 

 mar ckiana, a perpetual, true-breeding heterozygote. 



In Oe. Laniarckiana, alternate chromosomes normally pass to 

 opposite poles in a perfectly orderly manner. Catcheside and 

 Ford have shown, however, that if there is any irregularity in the 

 way they arrange themselves on the metaphase plate, nondis- 

 junction may result. In the simplest case, three adjacent chro- 

 mosomes of the circle are oriented towards one pole at metaphase 

 and pass to that pole at anaphase, as follows (the velans chro- 

 mosomes are in bold-face type) : 



1-2 (3-4 7 6 — 65 — 5-8 1112 10-9 1314 



1-2 4-7 8-11 12-10 9-13 14-3) 



This arrangement produces one gamete with eight chromo- 

 somes (seven velans and one gaudens) ; the other has only six. 

 The six-chromosome gamete will die, but the one with eight 

 chromosomes will survive. If this gamete mated with a gaudens 

 gamete, it would produce a trisomic of Oe. Lamarckiana, which 

 would show one extra chromosome tied in to the circle (Fig. 

 119). If it mated with a velans gamete, the chances are that 

 the zygote would be lethal. It is possible, however, that the 

 extra gaudens chromosome would carry the dominant allele of 

 the lethal velans gene. If so, we should expect five normal pairs 

 plus a figure-of-five consisting of two pairs tied together by a 

 univalent (Fig. 119). Similar configurations would take place 

 if the simple nondisjunction resulted in a gamete with seven 

 gaudens and one velans chromosomes, only now the large ring 

 would form if this gamete united with one bearing the velans 

 complex. Other more complicated types of nondisjunction could 

 also result from abnormal orientation in the chromosome circles, 

 and some very interesting configurations would be found in the 

 trisomic offspring of such plants. 



