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CHAPTER 22 



in triplicate is similarly fertilized, a female 

 would be produced having four of these 

 regions, three in one homolog and one in the 

 other. The question is: What would be the 

 phenotype of such a female? Could it make 

 any phenotypic difference whether these 

 regions are positioned two by two (as in 

 homozygous Bar) or three by one? Note 

 that the genie neighbors of the four regions 

 are different, when two regions are present 

 on each homolog, from what they are when 

 one homolog has three regions and the other 

 has one region. Knowing that position ef- 

 fect does occur, it is possible that the differ- 

 ences in neighbors, when these four regions 

 are arranged in the two different ways, could 

 make for different phenotypic effects. 



While we do not know precisely what the 

 potential position effect phenotype will be, 

 we can direct our attention to the number of 

 facets in the eye and look for any change 

 from the number expected. The normal 

 round eye of females and males (+/+ and 

 + /Y) contains more than 200 facets, or 

 ommatidia. The homozygous Bar female 

 {B/B) and hemizygous Bar male (5/Y) have 

 about 68 ommatidia per eye. The hetero- 

 zygous female (+/5) has about 150, Bar on 

 one chromosome being incompletely domi- 

 nant to the + condition in the homolog. 

 From the cross +/Y cT X B/B 9, then, the 

 usual Fi females are -\-/B with about 150 

 ommatidia per eye. Reversions to the + 

 condition, by means of crossing over in an 

 aneusynaptic tetrad, could be detected as a 

 +/+ female, as mentioned. The comple- 

 mentary crossover chromosome having the 



triple region together with a normal chromo- 

 some would produce a female which might 

 have less than 68 facets, or more than 68 

 but less than 150 facets, per compound eye. 



The experimental design is not yet com- 

 plete, however. Since we do not know how 

 often the chromosomes showing the poten- 

 tial position effect will be produced in meiosis, 

 we must eliminate two other possible causes 

 of change in eye shape which would be classi- 

 fied as exceptional. If the cross made is 

 + /Y X B/B, a sperm carrying two X's 

 (because of nondisjunction in the father) fer- 

 tilizing an egg with no X (because of non- 

 disjunction in the female) will produce a 

 + /+ female which would be counted as one 

 of the exceptional types we are seeking. Such 

 zygotes would be extremely rare. Never- 

 theless, we would be able to recognize them 

 if the + chromosome carried the gene for 

 yellow body color, y, as a marker, since such 

 nondisjunctionally produced females would 

 be yellow, not gray, in body color. (In this 

 way we would also recognize any female 

 progeny resulting from the contamination of 

 our cultures by flies of the yellow stock.) 

 So the cross we should make is y +^/Y 

 X >"+ Bly"^ B. For clarity, the genie symbol 

 for round eye is now given as +^. 



The other event we must eliminate from 

 confusing our results is mutation at or near 

 the B locus. The exceptional phenotypes 

 (round and an unknown shape of eye) we 

 are looking for would always be produced 

 following a crossing over in the region of 

 Bar. All we need do is to make the B/B 

 female dihybrid for genes near to, and on 



FIGURE 22-2. Compound 

 eye of Drosophila. 

 Left: Ultrabar; 

 center: Bar; 

 right: normal. 



