296 



ANIMAL BIOLOGY 



it in slightly different form and thus emphasize the essential facts 

 thus far discussed chiefly on the basis of Mendel's own work. 



Every cell of the soma of an individual may be regarded as 

 bearing a pair of genes for each alternative character (e.g., size in 





SPR 



sPR 



SPr 



/guy ,/' / ' V« 



Spr 



sPr apr 



Fig. 185. — The eight phenotypically different kinds of Guinea-pigs in 

 the F 2 generation of a trihybrid. S = short hair, s = long hair, P = pig- 

 mented coat, p = non-pigmented coat or albino, R = rough coat, r = smooth 

 coat. The hybrid parents (Fi) were phenotypically SPR. 



the case of the garden Pea), one member of each pair having been 

 derived from each gamete which contributed to the individual's 

 make-up. When both genes are identical {e.g., either SS or ss) 

 they are expressed in the soma (e.g., the plant is tall or dwarf). 

 The individual is homozygous with respect to size. But when the 

 two genes are not identical (e.g., Ss), the one, the dominant (S), 



