With such material numerous exercises may be con- 

 ducted, as follows: — 



1. Cross normal with vestigial winged forms, and 

 "carry on" through the F2 generation. 



2. Cross a normal fly with a black-bodied fly, and | 

 "carry on" through the F2 generation. 



3. Make reciprocal crossings of gray and yellow flies 

 and "carry on" through the F2 generation. , 



4. Make reciprocal crosses of a white-eyed male, if 

 such can be found, with a red-eyed female and I 

 "carry on ' ' through Fo 



Etc. 



Give the interpretation of the results. What was 

 Morgan's method of locating the 150 distinct characters in 

 Drosophila into four groups ? (See Morgan's Critiqued. 



Drosophila is readily and conveniently reared in bottles 

 of banana-agar inoculated with yeast. Two or three over- 

 ripe bananas are mashed in 200 cc water and added to 200 cc 

 of liquid agar, made by adding 4 grams of agar to 200 cc 

 of water and autoclaving until the agar is thoroughly dis- 

 solved. The banana-agar is then poured into 6 oz. cotton- 

 plugged, wide-mouthed bottles to a depth of half an inch. 



On solidification two or three drops of yeast solution 

 are added to each bottle. For student-work, test-tubes 

 or vials may be used. »^ 



The sexes of Drosophila may be distinguished by the 

 following characters of the abdomen: — 



Male Female 



(1) Three narrow bands (1) Five black bands, 

 and a black tip. 



(2) Black caudal band ex- (2) Black caudal band 

 tends around the under never meets on the 

 side. under surface. 



(3) Caudal extremity ra- (3) Caudal extremity 

 ther round and blunt. sharp and protruding. 



{Con^\x\t Gmetics Laboratory Manual by Babcock and 

 Collins for details regarding the method of breeding and 

 experimenting with Drosophila). 



Crossing-over 



Reference has already been made to the fact that the 

 expected Mendelian ratios in segregation do not always 

 occur. Such departures are due in many cases to linkage 

 of factors in the same chromosome, and in some cases to 

 "crossing-over" of factors from one of the homologous 

 chromosomes to the other at synapsis preparatory to re- 



121 



