422 



Aneuploids and Nondisjunction 



gene. If a normal, recessive female plant is crossed with a male 

 which is trisomic and has two dominant and one recessive al- 

 leles, the elimination of n -j- 1 male gametes will change the ratio 

 to 2 wild type : 1 recessive. If the female is the trisomic type 



Orientation of 

 trivalent 



Gametes 



Offspring of 

 Backcross 



Phenotype of 

 Offspring 





A 



Trisomic 

 dominant 



Normal 

 dominant 



Normal 

 dominant 



a 



I ■ 



Trisomic 

 dominant 



Trisomic 

 dominant 



a 



I t 



Normal 

 recessive 



Fig. 116. Inheritance in a trisomic with the genetic constitution AAa. 

 The three possible orientations of this trisomic at metaphase (first column) 

 and the six possible gametes that result (second column). The third col- 

 umn shows the genotypes and the fourth column the phenotypes formed 

 when an organism with this trivalent is backcrossed to the recessive. A 

 ratio of 5 dominant : 1 recessive would be found. 



and the male the normal recessive, the elimination of some of 

 the n + 1 female gametophytes and gametes will reduce the per- 

 centage of trisomic types in comparison with the percentage 

 found for eyeless where there is no elimination. The actual ratio 

 will depend upon the percentage of gametophytes eliminated and 

 will vary between 2:1, where all n + 1 gametes are eliminated, 

 to 5 : 1, where there is no elimination. 



Although we have assumed so far that the extra chromosome 

 is identical with one of the normal chromosomes in a genome, this 



