418 



Aneuploids and Nondisjunction 



or of the letter "Y," or finally of a ring bivalent with the third 

 chromosome attached to the ring at each end. 



When the chromosomes orient themselves on the spindle of 

 the first meiotic division, the trivalent takes a position on the 



Pachytene 



Diplotene 



Diakinesis 

 (with complete terminalization) 





Fig. 114. Trivalent configurations. A chain of three, a ring and rod, 

 three chromosomes joined together at each end, and a Y-shaped trivalent 

 are illustrated at diakinesis. The possible pairing arrangements at pachy- 

 tene and the formation of chiasmata in the paired segments at diplotene 

 are indicated. It is assumed that terminalization is complete. 



equator with the bivalents and adjusts itself so that the centro- 

 meres point towards the poles as well as they can. At first ana- 

 phase, regardless of which of the four types the trivalent has 

 assumed, two of the three homologous chromosomes go to one 

 pole and the third goes to the other; it is purely a matter of 

 chance to which pole the extra chromosome is transmitted. After 

 meiosis, two of the gametes or spores will be normal haploid 

 structures whereas the others will have n -\- 1 chromosomes and, 

 if they are viable, will be able to transmit the trisomic condition. 



