Position Effect and A 1 lei ism in Drosophila 



187 



females, proper pairing (eusynapsis) could 

 also occur and normal crossing over follow; 

 but this case potentially offers a different se- 

 quence of events in which synapsis occurs 

 incorrectly (aneusynapsis), the left region in 

 one chromosome pairing with the right region 

 of the second (as in Figure 22-1), leaving the 

 other two regions unpaired. If this incorrect 

 synapsis is followed by normal crossing over 

 anywhere in the paired region (as shown be- 

 tween 2 and 3 in the Figure), the crossover 

 strands will be 123456 and 123456 123456 



123456; the former strand has this region 

 only once and will therefore be normal (+), 

 while the latter has this region thrice. If an 

 egg containing the crossover with One region 

 is fertilized by an X-bearing sperm of a nor- 

 mal-eyed male, the zygote would produce a 

 daughter having normal eye shape, demon- 

 strating that Bar has reverted to +. This 

 could also be checked in a subsequent gen- 

 eration by examination of the salivary gland 

 chromosomes. 



If an egg containing an X with this region 



FIGURE 22-1. Diagrammatic representation of 

 t/ie normal and the Bar region of tlie X chromo- 

 some and the consequences of crossing over 

 following aneusynapsis. 



EUSYNAPSIS 



12 3 4 5 6 



12 3 4 5 6 



, PHENOTYPE 



K ji 



Normal 



Female 



(+/+) 



EUSYNAPSIS 



123456123456 



123456123456 



123456 12 3456 



ANEUSYNAPSIS 



\ 



I]X 



3456123456 



Homozygous 

 \ Bar Female 

 (B/B) 



Meiotic Products from 

 Crossing Over Indicated 



123456123456 



123456123456 123456 

 12 3 4 5 6 



Bar 

 ? 

 Normal 

 Bar 



