FIXING GOOD TRAITS 



Their progeny cannot be black because their 

 germ-plasm contains no hereditary factor for 

 blackness; nor can their progeny be rough-coated, 

 because their germ-plasm contains no hereditary 

 factors for rough-coatedness. 



Yet side by side with this new fixed race of 

 smooth-coated white guinea pigs, there are, as 

 we have seen, twins of the same fraternity that 

 instead of being white and smooth in color, are 

 black and rough. And these also constitute a 

 new race that will breed true because they 

 contain, as regards the unit character for color 

 and for condition of hair, only the dominant 

 factors of blackness and roughness. They also 

 are "homozygotes," but they are of the opposite 

 type — dominants instead of recessives. 



Meantinle, we must not overlook the other 

 members of the fraternity, twin brethren of these 

 new races, which are individually black and 

 rough, but which are "heterozygotes" as regards 

 the unit characters under consideration, and hence 

 will show progeny of variously mixed character- 

 istics as to roughness or smoothness of coat, and 

 as to black or white color. 



This illustration, perhaps, gives as tangible an 

 impression as can well be gained, of the com- 

 plexities that confront the experimenter when he 

 attempts to fix a new type of animal or plant. 



[245] 



