14 Kesearch;in Animal Brbs^dikg. , [Ape., 



Fl animals. If it is a " red " ovum it is also equally likely 

 to be fertilised by a " black " or a " red " sperm. In the 

 former case it will give a black of similar nature to the Fl animal ; 

 in the latter case it will produce a red. In considering the caU 

 thrown by an Fl cow mated to an Fl bull, the possibihty of its 

 being red is 1 in 4, of its being a true-breeding black 1 in 4, 

 and of its being an impure black (i.e., a black that produces both 

 " black " and " red " germ cells) is 2 in 4. If a large F2 gene- 

 ration from a number of Fl cows mated to Fl bulls were raised, 

 we should expect the F2 generation to consist of blacks and 

 reds in the proportion 3:1; further, of the blacks, only 1 out 

 of 3 would breed true to black in the sense of producing only 

 black germ cells. The others would act like the Fl parents and 

 throw about 25 per cent, of reds if mated together. 



The truth of Mendel's interpretation can be further tested by 

 mating what are called " back-crosses," i.e., by mating the 

 Fl animals back to the parents. Suppose, as is shown on the 

 left of Fig. 1 that the Fl cow is mated to the pure black bull. 

 As the cow's germ cells are of 2 kinds, " red " and " black," 

 and those of the bull are all black, we can obtain only two sorts 

 of animals, viz., those formed by the union of a " black " ovum 

 with a " black " sperm, and those formed by the union of a 

 " red " ovum with a black " sperm. The progeny will be all 

 black in appearance, but while half of them are true-breeding 

 blacks the other half will be capable of throw^ing reds when 

 suitably mated, ilgain, if the Fl animal is mated with the reces- 

 sive red as shov^n on the right side of Fig. 1, the germ cells of 

 the Fl being " black " and " red " in equal numbers, and the 

 germ cells of the recessive being " red," red and black among 

 the calves would be obtained in equal proportion. Moreover, all 

 the blacks so produced would be of the same constitution, i.e., 

 they would have the same output of germ cells as the F parent. 

 No true-breeding black would come from such mating. 



For the information of the breeder, the substance of the 

 matter is that when a definite pair of alternative characters is 

 being dealt with, of which one is dominant and the other reces- 

 sive, only three classes of animals are possible : (1) the animal 

 produced by the two like germ cells, both carrying the factor 

 for the dominant character : (2) the animal produced by two like 

 germ cells, both carrying the factor for the recessive character; 

 and (3) the animal produced by two unlike germ cells, one 

 of which carries the factor for the dominant, and the other 

 for the recessive character. (1) is the true-breeding dominant, 

 i2) the true-brepding recessive, and (S) the impure dominant, 



