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 F 2 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 Mendelian scheme of inheritance, 



