1921.] 



Research in Animal Breeding. 



13 



have been reared, it will be found that the blacks are about 

 3 times as numerous as the reds. To cover such facts Mendel 

 devised a simple explanation in terms of germ cells. Red and 

 black are alternative m heredity because they are alternative in 

 the germ cells. A germ cell contains eitJier that which causes the 

 development of black pigment or something which causes the 

 development of red pigment; but it is in the order of nature 

 that it cannot contain both. It is not known at present what 

 these contents are,, but as their existence is recognised a 

 name must be given to them, and they are usually spoken of 

 as factors. A germ cell, in our cattle then, contains oith-jr the 

 factor for black or the factor for red. When an animal breeds 

 true to a given character it means that all its germ cells carry 

 the factor for producing that character. All the germ cells 

 of a true breeding black contain the factor for black, and all 

 the germ cells of a true breeding red contain the factor for red. 



Jjct us refer again to the diagram (Fig. 1). If a red cow is 

 crossed with a black bull a " red " germ cell from the cow is 

 bein.^^ united with a " black " germ cell from the bull. 

 The resultant animal will be black because black is com- 

 pletely dominant over red, but although it is black it is 

 not a true breeding black. When such an animal reaches 

 maturity it produces germ cells corresponding to the germ 

 cells by which it was produced itself. In their formation 

 the red and the black factors separate cleanly from one 

 another, and in consequence half of its germ cells contain 

 the black factor and the other half contain the red factor. The 

 Fl animals, therefore, whether bulls or cows, produce " red " 

 and " black " germ cells in equal numbers, but owing to the 

 complete dominance of black, they are indistinguishable from 

 true-breeding blacks in appearance. Their genetical con- 

 stitution, as indicated by the output of germ cells, is very 

 different. The nature of the germ cells produced is diagram- 

 matica|ly represented in Fig. 1 by the contents of the white 

 circlps on each animal. When two Fl animals are mated, two 

 similar series of germ cells, each consisting of equal numbers of 

 ** r'^d " and " black," are brought together. 



Normally only a single ovum of the series produced by any 

 ind'V'd'^nl cow will be fertilised, but the probabilities are equal 

 of HiiQ b"ing a '* red " or a " black " ovum. If it is a *' black " 

 ovnni it is enunlly likely to be fertilised by a " black " or 

 a " red " sperm. In the former case it will give a true-breeding 

 hhfk : in the latter it will give a black of similar nature to the 



