Position Effect and AUelism in Drosophila 



189 



either side of, B, near enough (closer together 

 than ten crossover units) so that no double 

 crossovers could occur between them. Bar 

 is located at 57.0, forked bristles (/) at 56.7, 

 and carnation eye color (car) at 62.5 on the 

 X chromosome linkage map. We, therefore, 

 construct females that are 



y+f+ B car/y+fB car+. 



Any unusual eye shape that is noncrossover 

 between the loci for /and car can be elimi- 

 nated immediately from consideration. All 

 exceptional phenotypes of interest will be 

 crossovers between /and car; crossovers in 

 this region will normally be present in 5.8% 

 (62.5-56.7) of Fi daughters. Of course, the 

 males used will now have to be yf-{-^ car'Y 

 in order to identify the crossover daughters 

 (which will be either nonforked noncarna- 

 tion, or forked carnation). The cross then is: 

 7/ + S car, Y c^ X >'+/+ B car/y^fBcar+ 9 . 

 When the experiment is performed it is 

 found that about one female in two thousand 

 is round-eyed and carries a crossover be- 

 tween / and car; a similar number of fe- 

 males, that are crossovers in this region, have 

 very narrow eyes, called Ultrahar (Figure 22- 

 2), each eye containing about 45 facets. 

 Note that the reciprocal types of exceptional 

 flies are equally frequent, as would be ex- 

 pected if they were the reciprocal products of 

 a crossing over in an aneusynaptic tetrad. 

 Moreover, Ultrabar females can be bred, and 

 the salivary glands of their Fi larvae prove 

 they contained the triple region in one X and 

 a single region in the other X, as predicted. 

 You might still maintain that the Ultrabar 

 individual is a mutant and not a position ef- 

 fect, the production of the mutation somehow 

 being dependent upon a simultaneously 

 occurring crossover. That this is not the case 

 can be demonstrated by taking the two dif- 

 ferent exceptional types of X, placing them 

 together in a female, and occasionally ob- 

 taining perfectly typical Bar chromosomes. 

 These are found to carry two regions pro- 



duced by crossing over between the single 

 region of one chromosome and the middle 

 region of the triple-dose homolog (Figure 

 22 3, on p. 190). 



We conclude, therefore, that four regions, 

 aligned in different positions, produce dif- 

 ferent phenotypes. The alignments were 

 shifted by crossing over and not by mutation- 

 producing chromosome breaks, as was the 

 case in the rearrangement experiments pre- 

 viously described. This discussion should 

 lead you to appreciate the fact that in the ad- 

 vancement of genetic knowledge, while the 

 theory and the preparations to test it experi- 

 mentally are often complicated as in physics, 

 the data obtained are simple and unambigu- 

 ous. 



One other possibility presents itself for the 

 detection of position effects using crossing 

 over. If the genotype of a Drosophila female 

 were y a b spl ;+ a b spl+, a crossover be- 

 tween a and b would produce no new posi- 

 tions of the a and b genes relative to each 

 other, and no position effect would be ex- 

 pected, or found. But, suppose the genotype 

 was y a+ b spl/y+ a 6+ spl+. In this case 

 both the a and b loci are heterozygous; the 

 mutants are on different homologs, being 

 "across" from each other, or in trans position 

 (Figure 22-4). If crossing over occurs be- 



CIS 



TRANS 

 + b 



a b a + 



FIGURE 22-4. Cis and trans posi- 

 tions for diliybrid linlced genes. 



tween these loci we obtain y a+ b+ spl-^ and 

 >'+ a b spl as the crossover chromosomes. If 

 these two crossover chromosomes were pres- 

 ent in the same individual, then both mutants 

 (a and b) would be together on one homolog 

 and their normal alleles both would be on the 

 other homolog, so that these genes now are 



