ITS 



CHAP MR 38 



CIS 



+ + 



ab 



TRANS 

 + b 



a + 



FIGURE 38-4. Cis and tram positions for di- 

 hybrid linked v<'"< ,, >- 



and b ) would be linear neighbors; in the 

 latter ease a and /' (and a { and b~ ) would 

 be linear neighbors. If the trans dihybrid 

 had one phenotypic effect, the cis dihybrid 

 obtained from it by crossing over had an- 

 other, and if, by crossing over, the cis form 

 reverted to the trans form and restored the 

 old phenotypic effect, position effect would 

 be considered proved. 



This cis-trans test for position effect 

 should have the best chance of yielding a 

 positive result when the two pairs of genes 

 involved are adjacent to or very close to 

 one another. If the genes are very close 

 together, crossing over will rarely occur be- 

 tween them; large numbers of progeny 

 would have to be scored to assure at least 

 one crossover. 



The members of the multiple allelic 

 series at the white locus on the X chromo- 

 some of Drosophila are all located at 

 approximately 1 .5 on the crossover map. Al- 

 though the hybrid composed of vv" (apricot) 

 and vv (white), vv"u\ produces pale apricot 

 eye color, this result does not prove that 

 w a and vv are alternatives of the same gene. 

 Suppose vv" and vv are mutants of separate 

 but similarly-acting genes located close to- 

 gether, one at position 1.49 (w" + ) and one 

 at 1.51 (vv" ). The crossover data, being 

 finite and somewhat variable, could acci- 

 dentally have placed them both at locus 1.5. 

 If w a * and vv+ are close but separate loci, 

 the trans dihybrid should yield the cis di- 

 hybrid by crossing over. 



To test whether vv"' and vv^ are sepa- 



rate loci. Drosophila females which carry 

 an attached-X chromosome with y w spl on 

 one arm and y vv" spl on the other are 

 bred. Recall that the use of attached-X's 

 permits the recovery of two of the four 

 strands involved in each crossing over (p. 

 122). The attached-X genetic system some- 

 times yields both complementary crossover 

 types in the same gamete. Figure 38-5 

 (left side) shows schematically a portion of 

 this attached-X as it would appear in the 

 tetrad stage at the time of the crossing over 

 and indicates the standard genetic map loca- 

 tion of the y and spl markers. When a fe- 

 male with pale (dilute) apricot eye color 

 and this chromosome is crossed with a Bar- 

 containing male, the non-Bar F, daughters 

 (who carry a paternally-derived Y) are usu- 

 ally noncrossovers and have pale apricot- 

 colored eyes like their mother. Crossovers 

 between the region containing the white lo- 

 cus and the centromere produce either white 

 or apricot daughters. Barring mutation, 

 these phenotypes would be the only ones 

 expected if vv" and vv were alternatives of 

 the same gene. 



But if vv"* is separate from vv + , the for- 

 mer can be distinguished by a new symbol, 

 apr + . If apr + lay to the left of vv+ (as 

 shown in the left portion of the figure), apr 

 and vv would each have its own -+- allele 

 in the other arm of the parental attached-X 

 and, consequently, the female parent would 

 have to be a trans heterozygote with respect 

 to these loci. A rare crossing over between 

 these loci would produce the crossover at- 

 tached-X shown at the right of the figure. 

 As a result, the two mutants would be in 

 the cis position. 



When large numbers of daughters from 

 the attached-X females are examined, sev- 

 eral are found to have dull-red eyes. It is 

 essential to determine whether these flies are 

 mutant or the result of a change from the 



'■'• Based on the work of E. B. Lewis. 



