210 



Miscellaneous Linkage Topics 



not exact at this locus, but it also explains another puzzling 

 point — why homozygous, dominant bar-eyed flies can occa- 

 sionally produce the recessive, nonbarred type. 



/- 



B- 



fu 



\ 



- + 



/-- 



f-- 



-fu 



fu 



I 



+ 



+- 



fu 



a b 



Fig. 66. Unequal crossing over at the bar locus of Drosophila melano- 

 gaster. Before the discovery of the giant salivary gland chromosomes the 

 bar locus was believed to be a gene. As shown in (a), it appeared that 

 unequal crossing over was often found so that chromatids were produced 

 with two bar genes and with none. It has since been shown that the bar 

 locus is a reduplicated segment of the X chromosome involving about six 

 bands. Pairing as in (6) would thus produce chromatids with three seg- 

 ments or with only one. Flies with three reduplicated segments are double 

 bar and those with one are bar reverted. 



Double Chiasmata 



In discussing double crossing over, it was shown that more 

 than one chiasma frequently occurred in a bivalent. For our 

 example we selected a case where the same two chromatids were 

 involved in each chiasma (Fig. 55). However, any two nonsister 

 chromatids may form a chiasma, and each chiasma is formed 

 entirely independently of every other. It follows, therefore, that 

 either the same or different chromatids may be concerned at 

 successive chiasmata. 



When the same two chromatids are involved in the second 

 chiasma as are involved in the first, the chiasmata are said to be 

 reciprocal (Fig. 67). The second chiasma restores the order 

 which was changed by the first chiasma, and two noncrossover 

 chromatids are produced. When both the chromatids con- 

 cerned with the second chiasma are different from the two in- 

 volved in the first chiasma, the chiasmata are said to be com- 

 plementary. Two chiasmata of this type would produce all 



