HEREDITY AND VARIATION 



Exercise 1. — Study of External Features of Drosophila. 



(a) The fruitfiy Drosophila can be used to study the inheritance 

 of obvious structural features. It is necessary to become familiar 

 with the differences between males and females and to recognize the 

 typical or wild-type qualities of the flies before beginning a breeding 

 experiment. Obtain in your watch glass or on a piece of paper a wild- 

 type male and female fly which have been etherized. Examine with 

 the handlens and identify the head with the conspicuous red eyes, the 

 thorax to which the single pair of wings is attached, and the abdomen 

 which is banded with black. The wings extend beyond the tip of the 

 abdomen when they are folded. In flies etherized too long the wings 

 will be fixed at right angles to the body; the length can be checked 

 on living flies in a culture bottle. The male fly is smaller than the 

 female and has a rounded abdomen at the tip of which the black bands 

 are wider than in the female and more conspicuous from the ventral 

 surface. The tip of the abdomen in the female is somewhat pointed 

 and a tuft of short bristles can usually be seen. More details of struc- 

 ture can be observed with a binocular microscope and, if available, 

 demonstrations should be examined. Can you identify males and 

 females as they move about in a culture bottle? Be careful not to 

 release flies in the laboratory. 



(b) As Drosophila has been bred under laboratory conditions 

 numerous variations from the wild-type stock have arisen. Examine 

 demonstrations of as many eye, wing, bristle, and body-color varia- 

 tions as may be available. Such characters reappear in the same 

 form generation after generation so long as both parents exhibit the 

 character in question. 



Exercise 2. — An Experiment in Monohybridization. 



(c) In order to gain some information about the mechanism of 

 heredity individuals that differ with respect to one character such as 

 length of wing, shape of wing, color of eye, or color of body can be 

 bred together. Obtain two culture bottles each of which contains a 

 pad of sterile medium composed of cornmeal, syrup, agar-agar, and 

 water; the piece of paper is provided for the fly's egg-laying. Care- 

 fully remove the cotton plug and, holding the plug between two fingers, 



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