246 



PRINCIPLES OF ANIMAL BIOLOGY 



generation of Fig. 193. Or the F2 parents might both be Bh, so that the 

 Fz generation would be 0/ three kinds, just hke the F^ generation in Fig. 

 193. 



The other two possible matings between F^ individuals are Bh X 66, 

 and BB X Bh. Crosses of these two types are often made between Fi 

 animals and their two parent varieties, and are then spoken of as hack 

 crosses. The results of the former cross, that of a heterozygous black 

 with an albino, are shown in Fig. 194. 



Fig. 194. — Diagram showing the result of mating a heterozygous black guinea-pig with 

 an albino. This result is one of the possibilities in the production of an F3 generation. 



The first formula in F3 is of course that of a heterozygous black, the 

 second that of an albino. 



The other remaining possible mating in F2 for the production of an F3 

 generation is that between a homozygous black (BB) and a heterozygous 

 black (Bb). The nature of the F3 in such a case may be determined from 

 Fig. 195. 



All of the F3 animals in that diagram are black, but half of them are 

 homozygous, half of them are heterozygous. 



Fig 195. — Diagram showing the result of mating a homozygous black and a heterozy- 

 gous black guinea-pig. This result is one of the possibilities in ths production of an F3 

 generation. 



It is not desirable to carry this account beyond the F^ generation. 

 The nature of the F4 and succeeding generations would depend on princi- 

 ples illustrated in the production of the F3. No new matings are possible. 



The actual results described in this account are not in themselves 

 important. But it is important that the student should be able, in the 

 case of any two parents whose hereditary formulas /or a single character 

 are known, to predict the nature and formulas of all of their progeny. 



