686 General and Applied Biology 



6. Monohybrid, Dihybrid, and Trihybrid Crosses. — Monohybrid 

 crosses are those in which only one pair of differentiating characters 

 are considered, dihybrid are those in which two pairs are considered, 

 and trihybrid are those in which three pairs of different genes are in- 

 volved. 



By using the guinea pig and the following characters with their sym- 

 bols, the different crosses suggested above may be illustrated. 



B, black hair 

 b, white hair 

 R, rough hair 

 r, smooth hair 

 S, short hair 

 s, long hair 



When the dominant (ex- 

 pressed by the capital 

 letter) is present, it 

 expresses itself, even 

 though the opposite or 

 recessive be present. 



Monohybrid Cross (Guinea Pig) 

 Parents (P) 

 Gametes produced 

 Offspring (Fi) 

 Gametes for both sperms and eggs 



Black (BB) X White (bb) 



i i 



B b 



\ ^ 



Black (Bb) 



B b 



Sperm 



Offspring (F2) shown by the 



Punnet square or checkerboard 



B 



E 



g 

 g 

 s 



B 



b 



Thus we have produced 1 BB (black) 2 Bb 

 (black) and 1 bb (white) or a ratio of 

 3 black to 1 white. This is a phenotype 

 ratio of 3:1 for a monohybrid cross. 



X 



BLACK-SMOOTH 



BbRr 



BLACK-ROUCH 



WHITE-ROUGH 



X 



BLACK-ROUGH 



V 



F2 



o ay 



BLACK-ROUGH BLACK-SMOOTH 



WHITE-ROUGH 



WHITE-SMOOTH 



Fig. 336. — Dihybrid cross in guinea pigs when parents (P) are crossed. All 



of the Fi generation are black-rough. When the latter are intercrossed, the F2 



generation is produced as shown. The genes for the P and Fi generations are 

 shown. 



