176 



CHAPTER 38 



be recognized, however, if the • chromo- 

 some carried as a marker the gene for yellow 

 bodj color, y; such nondisjunctionall) pro- 

 duced daughters would be yellow, not gray, 



in body color. (In this wa\ we would also 

 be able to recognize any female progeny 

 resulting from cultures contaminated by flies 

 of the yellow stock.) Consequently, the 

 cross that should be made is y -\- B Y by 

 y+ B y ■ B. For clarity the gene symbol 

 for ovoid eye is now given as +". 



The other event that should not be al- 

 lowed to confuse the results is mutation at 

 or near the B locus. The exceptional pheno- 

 types sought (ovoid and unknown eye 

 shapes ) will always be produced after cross- 

 ing over in the region of Bar. The B B 

 female can be made dihybrid for genes near 

 and on either side of B — near enough (less 

 than ten crossover units apart) to avoid 

 double crossovers between them. On the X 

 chromosome linkage map Bar is located at 

 57.0; forked bristles (/) at 56.7; and carna- 

 tion eye color (car) at 62.5. Accordingly, 

 the females constructed are: 



y + /+ B car v + / B car + 



We can now eliminate from further con- 

 sideration any unusual eye shape that is non- 

 crossover between the loci for / and car. 

 All exceptional phenotypes of interest will be 

 crossovers between / and car; normally, 

 crossovers in this region will be present in 

 5.8% (62.5 minus 56.7) of F! daughters. 

 To identify the crossover daughters (which 



will be either nonlorkcd noncarnation or 

 forked carnation), the males used will have 

 to be yf -{-"car Y. The actual cross then 

 is: 



y / + " car Y 6 by 

 y+ f+ B car y f B car+ 9 



When the experiment is performed, about 

 one daughter in two thousand is ovoid-eyed 

 and carries a crossover between / and car; 

 a similar percentage of crossover daughters 

 have very narrow eyes, called Ultrabar (Fig- 

 ure 38-2), each eye containing about 45 

 facets. The two types of exceptional Hies 

 are equally frequent, as would be expected 

 of the reciprocal products of the hypothe- 

 sized crossing over. Moreover, Ultrabar 

 females contain a triple region in one X 

 and a single region in the other X, as pre- 

 dicted and revealed by examining the sali- 

 vary glands of their F\. Any argument that 

 the Ultrabar phenotype results from a 

 mutation — not a position effect — that is 

 somehow dependent upon a simultaneously- 

 occurring crossing over is disqualified by 

 obtaining females which carry both excep- 

 tional types of X and by occasionally find- 

 ing perfectly typical Bar chromosomes in 

 their progeny. These Bar chromosomes 

 prove to be the product of a crossing 

 over between the single region of one chro- 

 mosome and the middle region of the 

 triple-dose homolog (Figure 38-3). We 

 may conclude, therefore, that four regions 

 aligned in different ways — by crossing over 



figure 38-2. Compound 

 eye of Drosophila. 

 Left: Ultrabar; 



center: Bar; 

 right: normal. 



