192 



CHAPTER 22 



FIGURE 22-5. Crossing over between apricot 

 and white in attached-X chromosomes. 



+ apr + + 



locus and the centromere produce either 

 white daughters or apricot daughters. Bar- 

 ring mutation, these are the only phenotypes 

 expected if w" and h' are allelic. 



But if apricot is not allelic to w, it should 

 be distinguished by giving it a new kind of 

 symbol, apr. In this case, if apr lies to the 

 left of w, as shown in the left part of the 

 Figure, a rare crossover could occur between 

 these loci, as indicated there, producing the 

 crossover attached-X shown at the right of 

 the Figure. Note that if apr and w are non- 

 allelic, each must have its own + allele in 

 the other arm of the parental attached-X, so 

 that these female parents are trans heterozy- 

 gotes with respect to these loci. As a result 

 of the crossing over mentioned, these non- 

 allelic genes would be placed in the cis posi- 

 tion. If the cis position also produces light 

 apricot eye color, we will not be able to dis- 

 tinguish such a crossover from any other one 

 between y and spl that does not occur be- 

 tween apr and w, and we will be forced to 

 conclude (for lack of evidence) that apr and 

 w are really alleles. (Two genes are con- 

 sidered allelic, then, when they actually are 

 allelic and cannot be recombined by crossing 

 over, and when they are nonallelic and under- 

 go crossing over but give the same phenotype 

 in both the cis and trans positions.) 



When large numbers of daughters from 

 the attached-X females were examined, six 

 were found with dull-red eyes. To deter- 

 mine whether these flies were mutant or the 



H h 



H h 



apr w 



> 



spl 



result of a change from the trans to the cis 

 form, the attached-X's found in the dull-red- 

 eyed exceptional flies were detached (by col- 

 lecting the products of the occasional cross- 

 ing over that occurs between the attached-X 

 and the Y in the heterochromatic regions near 

 their centromeres). The genes carried in each 

 detached arm were determined and it was 

 found that one arm always could be repre- 

 sented as y apr+ vv+ 57;/+ and the other arm 

 always as >'+ apr w spl. Such results off"er 

 strong support for the view that the dull-red 

 exceptional females were cis heterozygotes 

 which produced dull-red eye color; they 

 also demonstrate that if apr and w are sepa- 

 rate loci, apr lies to the left of w on the X, 

 as shown in the Figure. (You should work 

 out the distribution of the markers following 

 crossing over between apr and vv on the as- 

 sumption that apr is to the right of w.) 



Proof that the exceptional dull-red females 

 were the result of a position eff"ect rather 

 than some mutational phenomenon was com- 

 pleted by mating these exceptional females 

 and obtaining occasional daughters which 

 were pale apricot. These new exceptional 

 daughters were then shown to contain the 

 original gene arrangement, this order having 

 been restored by crossing over. 



The phenotypic diff"erence between pale 

 apricot and dull red is undoubtedly the result 

 of position effect, since the only diff"erence 

 between the cis and trans conditions is the 

 arrangement which the same genetic material 



