RELATING TO SECONDARY SEXUAL CHARACTERS. 65 



contributes to the development of the horns. The other race, the 

 Suffolks, have no horns in either sex. Castration produces no change 

 in their hornless condition. 



When a Dorset ram is crossed to a Suffolk ewe the sons have horns, 

 the daughters lack them. The reciprocal cross gives the same results. 

 The factor or factors involved are therefore not sex-linked. When 

 the FI'S from the cross or from its reciprocal are inbred, four classes 

 of offspring are produced, namely: Horned male, 3; hornless male, 1; 

 horned female, 1 ; hornless female, 3. The ratios, as above, are approxi- 

 mately 3 : 1 : 1 : 3. 



A simple Mendelian explanation covers the results. If we assume 

 that the Dorsets, both male and female, are homozygous in a factor 

 for horns, H, that is not in the sex chromosome, and that the Suffolks 

 "lack this factor," i. e., that they have an allelemorphic factor for 

 hornlessness, the germ-cells are H-H and h-h, respectively. Only 

 one kind of individual, Hh, results in FI. Since the male with this 

 formula develops horns, we must conclude that the presence of the 

 testis (through its secretions) causes horns to develop, while in the 

 female of this same composition horns are not produced because of the 

 absence of the testes. The sex-cells in these FI individuals are H-h 

 and H-h. Chance meeting of these gametes will give 3 classes of 

 individuals, irrespective of sex, namely, (1) HH, (2) Hh, (1) hh. The 

 expectation for the males is that those of the composition (1) HH and 

 (2) Hh will develop horns, while those of the composition hh will not 

 develop horns. There should be 3 horned to 1 hornless male. In the 

 females we expect those with the composition (1) HH to develop 

 horns, since they have the same formula as the pure Dorset; those with 

 the formula Hh are not expected to develop horns, because the FI females 

 of this composition do not have horns; those with the formula hh are 

 not expected to develop horns, because they have the same composition 

 as have the pure Suffolk. There should be 3 hornless to 1 horned 

 female. Combining both sexes, the expectation for F 2 is 4 horned to 

 4 hornless. Arranged according to sex, these give the classes realized: 

 Horned male, 3; hornless male, 1; horned female, 1; hornless female, 3. 

 That this is the correct explanation is borne out by back-crossing the 

 hornless FI female to a hornless Suffolk ram. The former has two 

 kinds of gametes, H and h, the latter only gametes that bear the h 

 factor. Half the sons should be horned, half hornless, because half of 

 them are Hh and half hh. But none of the daughters should have 

 horns, because neither the Hh nor the hh females produce horns. This 

 is the result realized, viz, 3 hornless offspring to 1 horned. 



The preceding account of the inheritance of the factor for horns is 

 based on the combination of Dorsets and Suffolks used by Wood. 

 That other conditions may exist in other breeds and even in races of 



