V INTERACTION OF FACTORS 53 



Their potentialities may be quite different, although they 

 all look alike, but this can only be tested by treating them 

 with a colour developer. In the case of the mice and rab- 

 bits the potentiality for which we wish to test is the pres- 

 ence or absence of the factor G, and in order to develop 

 the colour we must introduce the factor C. Our de- 

 veloper, therefore, must contain C but not G. In other 

 words, it must be a homozygous black mouse or rabbit, 

 ggCCBB. ^Since such an animal is pure for C it must, 

 when mated with any of the albinos, produce only col- 

 oured offspring. And since it does not contain G the ap- 

 pearance of agoutis among its offspring must be attrib- 

 uted to the presence of G in the albino. Tested in this 

 way the F2 albinos were proved, as was expected, to be of 

 three kinds : (i) those which gave only agouti, i.e. which 

 were homozygous for G; (2) those which gave agoutis 

 and blacks in approximately equal numbers, i.e. which 

 were heterozygous for G ; and (3) those which gave only 

 blacks, and therefore did not contain G. 



Though albinos, whether mice, rabbits, rats, or other 

 animals, breed true to albinism, and though albinism be- 

 haves as a simple recessive to colour, yet albinos may be 

 of many different sorts. There are in fact just as many 

 kinds of albinos as there are coloured forms — neither 

 more nor less. And all these different kinds of albinos 

 may breed together, transmitting the various colour fac- 

 tors according to the MendeUan scheme of inheritance, 



