360 



CELL HEREDITY 



Pattern A 

 association of one 

 recombinant and 

 one nonrecombinant 

 chromosome 



21 units 45 units Pattern B 



I II association of 



y + both recombinant 



■■■■^■■■■■■■■■■H^C) a chromosomes 



cr 



Single 

 crossing 

 over in 

 region I 



+ 



czn 



ar 





Z)cf 



Wild 



IDa 



Z)d 



Wild 



DC 



K)b 



Yellow 



Singed + 



bcnz 



+ 



dcnz 



sn 



Single 

 crossing 



over in 

 region II 



Wild 



Ca 



Z)d 



Wild 

 — nc 



Wild 



+ 



c cm 



d d 



Z) 



Two-strand 

 J) double 

 crossing 

 over in + 



regions CZI 

 I and II 



Wild 



FIGURE 12.2. Somatic crossing over in the X chromosome of Drosophila (offer 

 Stern, 1936, Genefics, 21:625). 



to opposite poles. Unlike meiosis, this process leads to no reduction 

 in number. 



The possibilities for chromosome separation are shown in Figure 12.2; 

 only one arrangement of four will give rise to homozygosity of a given 

 recessive gene after each kind of crossing over. If the chromosomes 

 separate in pattern A after crossing over in region II, daughter nuclei 

 result, each one homozygous for a different locus, and twin spots are a 

 consequence. The size of these spots, which is not random, is a function of 

 the time in development when somatic crossing over occurred and of the 



