1921.] 



Research in Animal Breeding. 



113 



racters found in another breed, and with this in view it is clear 

 that Mendelian principles can be of great assistance to the 

 breeder. Let us suppose that the breeder wishes to obtain a 

 strain of red polled cattle out of Aberdeen Angus and Bed Short- 

 horn; and further, that he is not aware of the fact that polled 

 and horned, and black and red behave as simple Mendelian pairs. 

 From this crossing only polled blacks result. So far he can 

 say only that the Angus is prepotent ; but as soon as he raises a 

 F2 generation and views it in the light of Mendelian knowledge, 

 he begins to understand what is happening. The fact that he gets 

 no intermediates, but only blacks and reds in the F2 generation, 

 and that the latter form about one quarter of the total, tells him 

 that these colours depend upon a single pair of factors. From 

 the point of view of horns in the F2 generation, it will be noted 

 that polled and horned appear in the ratio 3:1; and this tells 

 the breeder that he is dealing with another pair bf factors. More- 

 over, the 9:3:3:1 ratio tells him that the two pairs are trans- 

 mitted independently ; for this is the meaning of the 9 : 3 : 3 : 1 

 ratio in analysing the phenomena of heredity. 



Having obtained the. polled reds the breeder wants to fix them 

 in the shortest time. To a problem of this sort Mendelian theory 

 may be a valuable guide. When once the factors concerned in a 

 cross have been determined, it is possible to calculate the propor- 

 tion of fixed animals in each class of the F2 generation, and to 

 suggest also how they are to be found. In explanation we may 

 examine the cattle cross from a slightly different point of view. 

 The nature of the F2 generation tells us that we are concerned 

 with a difference of two pairs of factors, viz., the pair for polled 

 and horned denoted by P and p, and the pair for black .and red, 

 B and b* It is clear that both dominants went in with the Angus 

 and both recessives with the Shorthorn : therefore, we denote the 

 polled black as PPBB and the Shorthorn as ppbb. Since the Fl 

 gets P and B from its Angus parent, and p and b from its Short- 

 horn parent, its constitution must be PpBb (Fig. 5). When Fl 

 forms germ cells each cell contains a representative of the P-p 

 pair, and also a representative of the B-b pair. Hence 50 per 

 cent, contain P and 50 per cent, contain p; also 50 per cent, 

 contain B and 50 per cent, contain b. Since fehe P2 generation 

 shows that the P-p pair and the B-b pair are transmitted inde- 

 pendently, we must suppose that any germ cell containing P 

 has an equal chance of containing either /> or &, and similarly , 

 any p germ cell has an equal chance of containing />' or b. Hence 



* It is customary to denote tl»o factor thai gives rise to the dominant of a 

 pair of alternative characters, by a capital letter, and thai upon w hich the 

 recessive depends, by the corresponding small tetter. 



B 



