532 



GENERAL CONSIDERATIONS 



duced by the union of the sex cells from these parents is the Fi (first filial) 

 generation; the second is the F2 (second filial) generation; and other 

 generations are labeled correspondingly. 



599. Checkerboard Diagrams. — The possibilities in the breeding 

 of hybrids may also be indicated in a diagram resembling a checkerboard, 

 which shows the gene constitution of sperm cells and egg cells and the 



Pare/7 fs 



P 



Gametes P 



m 



I 



B^ 



Pouffh Black smooth albino 

 RR BB rr bb 



Rough Black smao-th albino 

 RR BB rr bb 



? 



Rough Black 

 Rr Bb 



Rough Black 

 Rr Bb 



Gametes Of? Fj 



Rouah Black 

 m BB 



Rough Black 

 RR Bb 



Rough Black 

 Rr BB 



Rough Black 

 Rr Bb 



Rough Black 

 RR Bb 



Rough albino 

 RR bb 



Rough Black 

 Rr Bb 



Rough albino 

 Rr bb 



H. 



Rough Black 

 Rr BB 



Rough Black 

 Rr Bb 



smooth Black 

 rr BB 



smooth Black 

 rr Bb 



Rough Black 

 Rr Bb 



Rough cflbino 

 Rr bb 



smooth Black 

 rr Bb 



smooth albino 

 rr bb 



Fig. 325. — Diagram showing the results of crossing guinea pigs differing by two 

 characters, for each of which they are homozygous. The genotypic ratio in the F2 genera- 

 tion is 4:2:2:2:2:1:1:1:1; the phenotypic, 9:3:3:1. {From sketches by D. D. Whitney.) 



resulting possible combinations in the offspring. The accompanying 

 diagram (Fig. 324) would illustrate what would happen if the sex cells 

 differed in reference to a single pair of characters, black and albino, 

 represented by B and h. It is clear that in this case the result could be 

 expressed by a ratio 1 : 2:1, in which the first 1 is an individual homozygous 

 for the dominant character; the second 1 is an individual homozygous for 

 the recessive character; and the 2 is for the two individuals which are 

 heterozygous or hybrid for these characters. This is the genotypic ratio; 

 the phenotypic ratio would be 3:1, since three individuals, including the 



