RECENT DISCOVERIES IN HEREDITY. 201 



contain, unseen, the recessive character. Otherwise we may keep get- 

 ting mixed lots of offspring. The simplest and surest way of making 

 such a test is to mate the dominant animal with a recessive. For in 

 that case, if the dominant is pure, all the offspring will be dominants; 

 but if the dominant parent is a hybrid, half the offspring will be reces- 

 sives, half dominants. When the purity of two dominant animals of 

 opposite sex has once been established by breeding-test, we may use 

 them as the starting point of a race of dominants which we may be sure 

 will breed true. 



We must not, however, fall into the error of supposing that any 

 pair of dominants which produces only dominant offspring is, therefore, 

 pure. For progeny of this sort will be obtained if only one of the 

 parents is pure, the other being hybrid. In starting a race of domi- 

 nants which will breed true we must test each animal individually, by 

 mating it, preferably with a recessive, or else with a dominant known 

 to be hybrid in character. A test of the former sort should, as stated, 

 give 50 per cent, of recessive individuals if the dominant is impure; 

 a test of the latter sort should give 25 per cent, of recessives, if the 

 dominant is impure. Either sort of test should give only dominant 

 offspring if the dominant tested is pure. 



The statement has already been made that many characters are 

 independent of one another -in heredity; T hope now to demonstrate the 

 correctness of this idea m cases of alternative inheritance, even when 

 the independent characters relate to the same bodily parts. For this 

 purpose the coat-characters of guinea-pigs and rabbits are well adapted, 

 since they are exterior structures easily studied in the living animal. 

 I hope to show first that pigmentation of the hair is inherited quite 

 independently of its length, and secondly that hair-arrangement (in 

 smooth or rough coat) is inherited quite independently of both pig- 

 mentation and length of hair. 



When an ordinary short-haired guinea-pig (Fig. 5) is mated with 

 a long-haired albino guinea-pig (Fig. 7), all the young produced are 

 short-haired and pigmented, these being the dominant characters. (See 

 Fig. 8.) Xow if the cross-bred young are bred together, offspring of 

 four different sorts are produced. Two of the four sorts are identical 

 with the grandparents in character; they are short-haired pigmented 

 animals (Fig. 5) and long-haired albinos (Fig. 7), respectively. But 

 the other two sorts represent new combinations of characters ; they are 

 short-haired albinos (Fig. 9), and long-haired pigmented animals (Fig. 

 10). Further, these four sorts of individuals occur on the average in 

 definite numerical proportions, viz : 



9 short-haired pigmented animals, 

 3 shcrt-haired albino animals, 

 3 long-haired pigmented animals, and 

 1 long-haired albino. 



