228 



PRINCIPLES OF ANIMAL BIOLOGY 



made often, the latter seldom because its two kinds of offspring cannot 

 be distinguished. 



Two Pairs of Characters. — Since every animal possesses probably 

 thousands of different kinds of genes, any mating between individuals 

 serves as a test of the mode of inheritance of any or all characters in 

 which the two individuals differ. The experimenter may center his 

 attention on as many or as few of these as he wishes. For most purposes, 

 the smaller the number of characters studied simultaneously the better, 

 for the interpretations are clearer. No more than two pairs of characters 

 will be used in this book. For an example, Ave may add another pair of 

 characters in guinea pigs to the black-w^hite contrast already presented. 



Ordinary guinea pigs have smooth coats of hair, since the individual 

 hairs all slope in the same general direction in any part of the skin. 



Fig. 197. — Two guinea pigs with rough coats. The hairs are in many places arranged 

 in whorls, sloping away from the central point. {Courtesy of Professor W. E. Castle and the 

 Harvard University Press.) 



One variety, however, has a rough coat because, at a number of places 

 on the body, the hairs slope outward in all directions from a central 

 point like the radiating spokes of a wheel. These hairs push against 

 other hairs sloping in other directions, producing an unkempt appearance 

 (Fig. 197). Rough and smooth coat could be used in a single-pair cross, 

 in which case rough would appear in Fx, and the F2 would be three- 

 fourths rough, one-fourth smooth. That is, rough is dominant. 



The two pairs of characters, hair slope and color, can be combined 

 in four ways, namely, rough black, rough white (these two in Fig. 197), 

 smooth black, and smooth white (Fig. 193). To test the inheritance of 

 the two pairs of characters simultaneously, the animals crossed must 

 differ in both of them. Suppose one of the original parents is rough 

 black, the other smooth white. The Fi generation is rough black, since 

 these are the two dominant characters. When these hybrids, which are 

 heterozygous for both pairs of genes, produce their germ cells, the genes 

 of each pair separate from one another in the reduction division and go 

 to different cells. The two pairs undergo this separation independently, 



