Regulation of Gene Action — Position Effect in Drosophila 



All 



"Triple + " 



A 



/^\ 



y + f 



i i_ 



i r 



+ 



y f 



rR\ 



f a b c d e f a 



X 



f car 



ULTRABAR 

 FEMALE 



FIGURE 38-3. 



Production 



of Bar chromosomes 



by crossing over 



in Ultrabar females. 



"Triple + " 

 A 



y + \ + abcdef abcdef abcdef car 



+ ( \ 



abcdef abcdef 



V" f 



y + f 



i i_ 



i r 



y + f 



abcdef abcdef 



abcdef 



MEIOTIC 

 PRODUCTS 



and not by mutation-producing chromo- 

 some breaks — produce different pheno- 

 types. 



From a B (double region) chromosome, 

 it is also possible to obtain a few chromo- 

 somes nonrecombinant for bordering mark- 

 ers but + (single region) or Ultrabar (tri- 

 ple region). This unusual circumstance is 

 brought about by an intrachromosomal ex- 

 change within the double loop that is formed 

 when the two members of the tandemly du- 

 plicated region synapse with each other. 

 Similarly, intrachromosomal exchange within 

 an Ultrabar chromosome can yield -f and 

 B chromosomes. - 



-See H. M. Peterson and J. R. Laughnan (1963). 



Another way of detecting position ef- 

 fects involving crossing over is possible. If 

 the genotype of a Drosophila female is 

 y a^b spl y + a b + spl + , both the a and b 

 loci are heterozygous and the mutants are 

 in different homologs or in trans position 

 (Figure 38-4). If crossing over occurs be- 

 tween these loci, the resulting crossover 

 chromosomes would be y a^ b + spl + and 

 y + a b spl. When both these crossover chro- 

 mosomes are present in the same individ- 

 ual, both mutants (a and b) are in the same 

 homolog or in cis position. Both the trans 

 and cis heterozygotes have the same number 

 of loci on each chromosome. In the for- 

 mer instance, however, a and b+ (and a + 



