Regulation of Gene Action — Position Effect in Drosophila 



475 



tial position effect phenotype should be, we 

 can look for any significant variation from 

 the number of eye facets expected. The 

 normal ovoid eye of females and males 

 ( + + and + Y ) contains more than 200 

 facets. The homozygous Bar female (B B) 

 and hemizygous Bar male (BY) have about 

 68 ommatidia per eye. The heterozygous 

 female (+ B) has about 150; Bar on one 

 chromosome is incompletely dominant to -f 

 in the homolog. From the cross -f-/Y 6 by 

 B B 9 , the typical F, females are + B with 

 about 150 ommatidia per eye. As men- 

 tioned earlier, reversions to the + condition 

 by crossing over in an obliquely synapsed 

 tetrad could be detected as a + -j- female. 

 The complementary crossover — a triple re- 



gion chromosome — with a normal chromo- 

 some would produce a female whose eye 

 might have less than 68 facets or have more 

 than 68 but less than 150 facets. 



The design of the experiment is not yet 

 complete, however. Since we do not know 

 how often a chromosome showing the po- 

 tential position effect will be produced in 

 meiosis, two other possible causes of excep- 

 tional eye shape must be eliminated. If the 

 cross made is -f Y by B fi, a sperm carry- 

 ing two X's (because of nondisjunction in 

 the father) that fertilizes an egg with no X 

 (because of nondisjunction in the mother) 

 will produce a +/+ daughter counted as 

 one of the exceptional types. Although such 

 zygotes would be extremely rare, they would 



bcdef 



a b c d e f 



PHENOTYPE 



Normal 



Female 



(+/+) 



abcdefa bcdef 



abcdefabcdef 



abcdefab cdef 



I 



"~v — 



a by\c d e f a 



Homozygous 

 S Bar Female 



(B/B) 



bcdef 



FIGURE 38-1. 



Diagrammatic representation 

 of the normal 

 and the Bar region 

 of the X chromosome 

 and the consequences 

 of crossing over after 

 oblique synapsis ( / ) . 



Meiotic Products from 

 Crossing Over Indicated 



abcdefabcdef 

 abcdef abcdefabcdef 

 a b c d e f 



abcdefabcdef 



Bar 

 ? 

 Normal 

 Bar 



