116 INTRODUCTION TO GENETICS OF MAN AND FRUIT FLY 



Exercise XXII 



The schedule for the entire experiment is as 



follows: 



This week: Remove the parent flies, following 



the directions below. 

 Week 2: Cross the F\ flies, and record their 



phenotypes. 

 Week 3: Remove the Fi parents. 

 Week 4: Score the results of this cross and of 



a more complex cross which will be 



given you. 



Each week you will find detailed instructions 

 for proceeding with the experiment. 



This week's work 



The main job this week, apart from removing 

 the adults from your vials, is to get to know the 

 flies and learn to handle them. 



Begin by removing the adults from the vials, 

 etherizing them as described below. Have the 

 vials ready beforehand to be stored for incuba- 

 tion. Be sure each vial is labeled with your name 

 and a description of the cross and the date it was 

 made. Each mutant gene has a special symbol, 

 dp for dumpy, e for ebony, and + for wild type. 

 We will use diploid formulas that represent the 

 somatic cells of the parents, rather than haploid 

 formulas that would represent mature germ cells. 

 The formula for the female should be written 

 on the left, followed by X and the formula for 

 the male. The pair of gene symbols for each 

 chromosome pair is written one above the other, 

 like a fraction; so for example, homozygous 

 dumpy is represented by dpjdp. The initial cross, 

 therefore, in which homozygous dumpy females 

 were mated with homozygous ebony males, can 

 be written dpjdp X eje. 



As soon as the adults have been taken out of 

 the vials, return the vials to the boxes so that 

 they can be incubated until next week. 



The etherized adults should be examined care- 

 fully under the dissecting microscope. You 

 should be able to distinguish males from females 

 and the mutant types from wild-type flies, several 

 of which will be provided for comparison. You 

 will find descriptions to guide you below and in 

 your reading. 



Examine also Drosophila eggs, larvae, and 

 pupae under the dissecting microscope. These 

 stages will be given you. Do not take any eggs 

 from your experiment. 



In any free time, examine the prepared slides 

 which are set up under the demonstration 

 microscopes. 



Life cycle 



At 25°C the entire life cycle of Drosophila 

 is usually completed within 10 days. It includes 

 four stages: egg, larva, pupa, and imago 

 (adult), as in all Diptera (true flies). The eggs, 

 about 0.5 mm in length, are sausage-shaped 

 white structures bearing a pair of filaments at 

 the end, which help to keep them from sinking 

 into the soft food on which the eggs are always 

 laid. The larvae are little white maggots which 

 burrow in the food at the bottom of the vials. 

 Drosophila larvae undergo two molts after 

 emerging from the egg; the larval period thus 

 consists of three stages (instars). Larvae may 

 be up to 4.5 mm long in the final stage; it is 

 from them that the giant salivary gland chromo- 

 somes are obtained. At the end of the third 

 instar, the larvae crawl up to a dry spot on the 

 wall of the container, where they pupate in 

 small dark cocoons. Pupation lasts about four 

 days at 25°C, after which the adult fruit fly 

 emerges. The adult is at first light in color, and 

 its wings are crumpled; but within a few hours 

 the wings expand and the adult takes on its 

 familiar appearance. 



Determining the sex of adults 



Males can be told from females with the 

 naked eye, using several dilTerent criteria. 

 Though the external genitalia are more com- 

 plex in males, this difference is difficult to see. 

 The abdomen of the female is long and pointed 

 at the end, whereas that of the male is consider- 

 ably shorter and somewhat stubby. Further- 

 more, the entire rear portion of the male 

 abdomen is black, whereas in the female dark 

 and light bands alternate to the tip. One of the 



