474 



( II M' I IK 3X 



of crossing over should exhibit the position 

 effect. It docs. 



In Drosophila, position effects often ac- 

 company rearrangements that bring genes in 

 euchromatin close to those in heterochro- 

 matin. Placing a gene normally located in 

 a dichromatic region near or in a hetero- 

 chromatic region often produces a special, 

 wavering position effect which is expressed 

 in the phenotype as a mosaic or variegated 

 characteristic. Thus, for example, if by 

 paracentric inversion, the gene for dull-red 

 eye color on the X chromosome, vv+ (nor- 

 mally located in euchromatin) is placed in 

 the heterochromatin near the centromere, 

 the result is mottled eye color, white and 

 dull-red speckles. Such variegation is re- 

 duced, however, if, by breeding, an extra Y 

 chromosome or another heterochromatin- 

 rich chromosome is added to the genotype. 

 It is not altogether clear yet how this sup- 

 pression of variegation is brought about. 



The only requirement for the occurrence 

 of position effect is an appropriate change 

 of a gene's linear neighbors. Breakage 

 merely provides a way of obtaining such 

 changes. Other mechanisms — such as cross- 

 ing over — which change the relative posi- 

 tions of genes should also reveal position 

 effects. Let us see if a crossover system ' 

 which will produce a position effect can be 

 devised. 



An X-linked mutant in Drosophila, Bar 

 (B), reduces the number of facets (om- 

 matidia ) in the compound eyes, thereby nar- 

 rowing the normally ovoid eye to a slit. 

 When the normal and the /tar-containing 

 chromosomes are studied in nuclei of larval 

 salivary glands, it is found that about seven 

 successive bands in the normal chromosome 

 are duplicated in tandem in the Bar chro- 

 mosome. Let us designate such a single 

 region as abedef. Consequently, a normal 



1 Based upon investigations of A. H. Sturtevant. 

 H. J. Muller. C. B. Bridges, and others. 



female contains abedef abedef and a homo- 

 zygous Bar female abedef abedef abedef 

 abedef. In normal (+ -f ) females, homol- 

 ogous letters (parts) of the two homologs 

 synapse and crossing over takes place be- 

 tween corresponding letters. In homozygous 

 Bar (B B) females, proper synapsis and 

 normal crossing over can also occur, but in 

 this case a potentially different sequence of 

 events will cause synapsis to occur incor- 

 rectly — the left region in one chromosome 

 will pair with the right region of the second 

 (Figure 38-1), leaving the other two re- 

 gions unsynapsed. If this oblique synapsis 

 is followed by normal crossing over any- 

 where in the paired region (as shown be- 

 tween b and c in the figure), the crossover 

 strands will be abedef and abedef abedef 

 abedef. The former strand has this region 

 only once — and will therefore be normal 

 ( + ) — whereas the latter has this region 

 three times. If an egg containing the one- 

 region crossover is fertilized by an X-bear- 

 ing sperm of a normal-eyed male, the zygote 

 will produce a daughter having normal eye 

 shape, thereby demonstrating that Bar has 

 reverted to + . This result can be checked 

 in a subsequent generation by examining the 

 salivary gland chromosomes. 



If an egg containing the three-region 

 crossover is similarly fertilized, a female will 

 be produced having four of these regions, 

 three in one homolog and one in the other. 

 What will be the phenotype of such a female? 

 Does it make any difference phenotypically 

 whether these regions are grouped two and 

 two (as in homozygous Bar) or three and 

 one? Note that the genie neighbors of the 

 four regions are different when two regions 

 are present on each homolog than they are 

 when one homolog has three regions and 

 the other has one. Since position effect 

 occurs, this gene neighbor difference may 

 result in different phenotypes. 



Although we do not know what the poten- 



