1921.] Research i\ Animal Breeding. 115 



the Fl animals will produce the 4 kinds of germ cells PB, Pb, pB 

 and pb, and produce them in equal numbers. When Fl beasts 

 arc mated together it means that we are bringing together a 

 series of ova of this nature with a similar series of sperms. The 

 simplest way to arrive at the result is to make a figure of 16 

 squares, as shown in Fig. 5, and to write the above series of 

 germ cells first across the four horizontal rows of the figure, and 

 then down the four vertical rows of the same figure. This will 

 give all the different possible combinations in the proportions in 

 which they may be expected to occur, i.e., the constitution of 

 the F2 generation. Examination of the squares shows that 9 out 

 of the 16 contain both P and D and are, therefore, polled blacks, 

 3 contain B and p, which are horned blacks; 3, in w 7 hich P is 

 associated with b, are polled reds; while 1, having only b and p, 

 must be red horned. The three red polled are not all alike. One 

 is PPbb and two are Ppbb (Fig. 5, squares numbered 6, 8, and 

 14). The former is pure for the polled factor, having received it 

 from both parents ; the others, however, have received it from 

 one parent only, and are consequently impure. The polled 

 character is already fixed in one -third of the red polled F2 beasts, 

 but how are the fixed to be distinguished from the unfixed beasts? 

 As pointed out in the previous article, this can be done by crossing 

 with the recessive, which in this case is the horned beast. The 

 fixed polled red PPbb gives only polled beasts when mated with 

 horned animals, the impure polled red Ppbb gives on the average 

 equal numbers of polled and horned. The above example w 7 as 

 selected as a very simple illustration of the manner in which the 

 " break-up " of the type, and the recombination of characters is 

 interpreted on Mendelian lines, but the general principle applies 

 to far more complicated cases. It has provided us with a simple 

 explanation of the curious phenomenon of reversion on crossing, 

 a phenomenon which has puzzled the practical breeder and the 

 man of science. As, however, no fresh principle is involved in such 

 cases there is no need to consider them here in further detail. 



Many of the characters of animals with which the breeder 

 deals, owe their manifestation to the presence of one or other 

 definite factor, which is transmitted according to a definite 

 scheme. If these factors are not divisible under normal rendi- 

 tions, they must be handed on through the germ cells as definite 

 entities producing their full effect in each successive generation. 

 Continual crossing of black with red does not diminish the 

 potency of the black factor. So long as it is passed on through ;i 

 germ cell it produces its full effect. Tin's relative permanence 

 of the factors, assuming it to be well founded, is doubly important 



b 2 



