XXV 



COMPLETION OF THE 

 GENETICS EXPERIMENT 



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The adult Fg flies in the dumpy-ebony experi- 

 ment have now emerged, and this laboratory 

 period will be devoted to examining them and 

 evaluating the results. Since the flies will not be 

 needed again, they may be overetherized before 

 counting. The more flies counted, the more 

 reliable the results will be. 



Eight possible classes of flies can be dis- 

 tinguished from the crosses you have made: 

 wild-type males, ebony males, dumpy males, 

 ebony-dumpy males, and the same four classes 

 of females. Determine the number of flies in 

 each of the eight categories. From this you can 

 tell whether the mutant genes are dominant or 

 recessive, linked or not, and sex-linked or not. 



After all the students have finished their 

 counts, all the results will be summed up to give 

 class totals, which can be treated as one large 

 experiment. 



Consider the dumpy : wild-type and ebony: 

 wild-type ratios separately. What are they? 



Make a diagram showing the genotypes of 

 members of the parental generation, the Fi 

 generation, and the Fa generation. 



CHROMOSOME MAPPING: 

 A THREE-GENE EXPERIMENT 



The dumpy-ebony cross has illustrated simple 

 segregation and independent assortment. To 

 demonstrate a more complex situation in 



Drosophlla genetics, you will work with a hatch- 

 ing generation in which three genes are segre- 

 gated: apricot, cut, and bar. Apricot refers to 

 the eye color which is much lighter than the 

 wild-type red; cut to a marginal cleft in the 

 wing; and bar to the eye shape: in the male the 

 eye is restricted to a narrow vertical bar and 

 in the female to a kidney-shaped slit. These 

 characteristics are easily spotted. The symbol 

 for apricot is w" (since apricot is an allele of 

 white); for cut it is ct, and B stands for bar. 



Examine your flies (you may overetherize 

 them) and record the various combinations of 

 these genes and wild-type genes separately for 

 the sexes. 



There are eight possible phenotypes: 



+ 



The parental types : 



+ B red eye, normal wing, bar eye 

 ct + apricot, cut wing, normal eye 



Single crossovers in region I. 



M'" ct B apricot, cut wing, bar eye 

 + + 4- comnletelv wilH-tvnp 



completely wild-type 



Single crossovers in region II: 



H" + B apricot, normal wing, bar eye 

 -\- ct + red eye, cut wing, normal eye 



Possible double crossovers: 



w" -f + apricot, normal wing, normal eye 

 -\- ct B red eye, cut wing, bar eye 



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