294 



ANIMAL BIOLOGY 



& 



,J[\^jMHk 



f: 



(■■ ■■-■ : 3 '•' '•■• 





&*::« 



F, 





,. A^. y/^ssfy 



**&* 



r ■ : ■. J- 



i •«<* 



3-s£u- 



%;j^ 





Fig. 183B. — Result of crossing pure smooth black with rough white Guinea- 

 pigs. Rough and black (pigmented) are dominant. Note that each parent 

 (P) bears both a dominant and a recessive character. The F 2 illustrates the 

 principle of independent assortment. 



3. Trihybrids 



Similarly, Mendelian trihybrids, for example the cross between 

 pure tall Peas bearing yellow round seeds and dwarfs bearing green 

 wrinkled seeds, or the cross between pure Guinea-pigs with long, 

 rough, black hair and those with short, smooth, white hair give 

 in the F2 generation 27 genotypes and 8 phenotypes; the number 

 of individuals in the phenotypes being in the ratio 27 : 9 : 9 : 9 : 

 3:3:3:1. Of course, in nature there are few instances in which 

 parents and offspring differ by only one, two, or three characters, 

 but since characters arising from each pair of allelomorphs can 

 usually be treated singly, convenience demands that the analysis 

 be made with respect to one or two pairs at a time, which there- 

 fore is the usual method of procedure. (Figs. 184, 185.) 



4. Summary 



Before passing to certain extensions of these established hered- 

 itary principles, it may serve to clarify the subject if we restate 



