84 



Monohybrids 



P, flies 



Pi gametes 



F, flies 



Fi gametes 



F, flies 



Homozygous Wild Type Curved 



CC cc 



/\ /\ 



© ©. © © 



Heterozygous 

 Cc 



/\ 

 © © 



Fi sperm 

 C c 



F. 



Eggs 



C 



c 



F2 genotypic ratio 

 F2 phenotypic ratio 



ICC ■ 2Cc : Ice 

 3 Wild Type: 1 Curved 



Backcross of Fi to 



Dominant Parent 



C C ^ Cc 



Backcross of Fj to 



Recessive Parent 



c c ^ Cc 



All Wild Type 



1 Wild Type: 1 Curved 



Fig. 28. The checkerboard method of determining the F2 and backcross 

 generation.?. A cross between a wild-type female {CO and curved 

 winged male {cc) produces a heterozygous wild type {Cc). The eggs of 

 the Fi are C or c as are the sperm. Since either kind of sperm can 

 unite with either kind of egg at random, four possible kinds of F2 indi- 

 viduals will be produced in equal numbers; but since two kinds are alike, 

 the Fo genotypic ratio will be ICC : 2Cc : Ice. Since dominance is com- 

 plete, the F2 phenotypic ratio will be 3 wild type : 1 curved. The back- 

 crosses of the Fi to the dominant and to the recessive parents are indi- 

 cated in the lower half of the figure. 



Backcross 



If an Fi fly is mated with one of its own parents, the cross is 

 called a backcross. If the Fi is backcrossed to the dominant 

 parent, all the ofTspring will be alike phenotypically although 

 genotypically half will be CC and half Cc. If the backcross is 

 made with the recessive parent, one half the offspring will be 

 genotypically Cc and the other half will be homozygous for the 



