Questions and Problems 



217 



normally gray body. Wherever such a somatic crossing over 

 took place a small yellow spot would be visible on the body of 

 the fly. Somatic crossing over, like pachytene crossing over, oc- 

 curs in a four-strand stage between two nonsister chromatids. 

 It does not, however, form chiasmata, or, if it does, the chias- 

 mata disappear before metaphase and do not affect the orienta- 

 tion of the chromosomes on the spindle. It does not, of course, 



No Crossover 



+ + 



Somatic 

 metaphase . 



y sn 



sn 



> 



y sn 

 y sny 



Crossover between y and sn 



+ + + + 



y sn 



> 



+ sn 



> 



y sn 



> 2II1J 



+ sn 



+ sn y or y + y y sn 



Resulting 

 cells. 



y sn 



Wild Type 



y sn 



Wild Type 



D ( 



y sn 



Wild Type 



y + 



+ sn 



+ sn 



Wild Type 



y + 



y sn 



Yellow 



Wild Type 



Fig. 68. Somatic crossing over between the loci of yellow and singed 

 in Drosophila melanogaster. For explanation, see text. 



have any effect on germinal tissue. Its results are immediate, 

 for daughter cells of the one in which the crossover occurred 

 will be phenotypically different from the normal cells, provided 

 that, as in this case, the part of the body affected is a part 

 under the control of the gene which crossed over. 



QUESTIONS AND PROBLEMS 



1. Show by diagrams how a heteromorphic bivalent might be used to 

 give evidence in support of either the classical or the partial chiasma- 

 type theories. Is the evidence positive or negative? 



2. If bar eye appears to be due to a reduplication of a small segment 

 of a chromosome, does this have any possible bearing on the nature or 

 existence of genes? 



3. Could somatic crossing over produce small areas with singed 

 bristle? Show by a diagram. 



