C H 
 431 
 A7 
 Biology 
 
 UC-NRLF 
 
 C 2 b72 373 
 
LIBRA 
 
 UNIVER 
 OF 
 
 CALIFO1 
 
 HORNS IN SHEEP AS A TYPICAL SEX-LIMI- 
 TED CHARACTER 
 
 T. R. ARKELL AND C. B. DAVENPORT 
 
 [Reprinted from SCIENCE, N. S., Vol. XXXV., No. 897, Pages 375-377, March 8, 
 

[Reprinted from SCIENCE, N. S., Vol. XXXV., No. 897, Pages 375-377, March S, 1912] 
 
 HORNS IN SHEEP AS A TYPICAL SEX-LIMITED CHARACTER 1 
 
 SEVERAL years ago Wood (1905) published 
 a note in -which, he showed that, in a cross be- 
 tween a Dorset Horn and a Suffolk (belong- 
 ing to a hornless breed of sheep), the male 
 offspring all developed horns but the female 
 offspring remained hornless. He showed 
 further that in the F 2 generation hornless 
 males arise, and these do not carry the de- 
 terminer for horns, and horned females, but 
 only when they have the determiner duplex. 
 Bateson (1909, p. 173) has discussed these 
 facts and drawn the conclusion : " Sex itself 
 acts as a specific interference, stopping or in- 
 hibiting the effects of a dominant factor, and 
 it is not a little remarkable that the inhibi- 
 tion occurs always, so far as we know, in the 
 female, never in the male." He admits, how- 
 ever, the difficulty in distinguishing between 
 this probability and the other possibility; viz., 
 that the male provides a stimulating factor. 
 Castle (1911, p. 102) concludes that the rea- 
 son horns are more strongly developed in 
 males than females is "the presence of the 
 male sex-gland in the body, or rather prob- 
 
 1 Joint contribution from the New Hampshire 
 Agricultural Experiment Station and the Station 
 for Experimental Evolution, Carnegie Institution 
 of Washington. 
 
 ably some substance given off into the blood 
 from the sex gland, favoring growth of the 
 horns " ; and he adds that if the male Merino 
 sheep (in which, usually, the male, and the 
 male only is horned) is castrated early in life 
 no horns are formed. He gives no reference 
 for the last statement; and in view of thep- 
 variability of the horned condition in the 
 males of the " Merinos " the conditions of the 
 experiments would have to be carefully con- 
 sidered before such a result could be accepted 
 as settling the question of the dependence of 
 horns in heterozygous males upon a secretion 
 from the testis. 
 
 The hypothesis that we have adopted and 
 which works with entire satisfaction assumes, 
 first, that, as in man so in sheep, the male is 
 heterozygous (simplex) in sex. One sex- 
 chromosome is then to be expected in the male, 
 and substantially this condition has been 
 found to hold for man by Guyer (1910). The 
 female will then be duplex in respect to sex. 
 One further assumption is necessary; there is 
 an inhibitor to horn formation, and this is 
 located on the sex chromosome; consequently 
 it is simplex in the male and duplex in the 
 female. Thus it belongs to the well-known 
 class of sex-limited characters. The inhibi- 
 
 272702 
 
Determiners in Germ Plasm of 
 
 No. of Horned and Hornless Offspring 
 
 Male 
 
 Female 
 
 Males 
 
 Females. 
 
 Horned 
 
 Hornless 
 
 Horned 
 
 Hornless 
 
 Soma Xxhhli (hornless) 
 
 XXhhll (hornless) 
 Xhl 
 
 XXhhll (hornless) 
 
 
 
 4 
 
 
 
 12 
 
 Gametes Xhl. 
 
 xhi 
 Zyjrotes Xxhhli (hornless) 
 
 
 fioma Xxhhli (hornless) 
 
 XXH h II (hornless, simplex) 
 XHI 
 Xhl 
 XXHhII( hornless) 
 XXhhll (hornless) 
 
 (2) 
 2 
 
 (2) 
 2 
 
 (0) 
 
 
 (2) 
 2 
 
 Gametes Xhl . 
 
 
 
 Xxhhli (hornless) 
 
 
 Soma Xxhhli (hornless) 
 
 XXHHII ( horned) 
 XHI 
 
 XXHhll (hornless) 
 
 15 
 
 
 
 
 
 24 
 
 Gametes Xhl. 
 
 xhi 
 
 
 Soma XxHhli (horned, simplex) 
 
 XXhhll ( hornless) 
 Xhl 
 
 XXHhll (hornless) 
 XXhhll ( hornless) 
 
 (.5) 
 
 
 (.5) 
 
 (0) 
 
 
 ( 1> 
 
 Xhl 
 xHi 
 xhi . 
 Zygotes XxHhli (horned) 
 
 Xxhhli ( hornless ) 
 
 
 Soma XxHhli (horned, simplex) 
 Gametes XHI 
 
 XXHhll (hornless, simplex) 
 XHI 
 Xhl 
 
 XXHHII (horned) 
 XXHhll) 
 XXHhll}- (hornless) 
 XXhhll J 
 
 (7.5) 
 6 
 
 (2.5) 
 
 4 
 
 (2) 
 
 (6) 
 7 
 
 Xhl 
 
 xHi 
 xhi 
 Zygotes XxHHIi} 
 
 XxHhli } (horned) 
 
 XxHhli j 
 
 Xxhhli (hornless) 
 
 
 Soma XxHhli (horned, simplex) 
 Gametes XHI 
 
 XXHHII (horned) 
 XHI 
 
 XXHHII (horned) 
 XXHhll (hornless) 
 
 (10) 
 10 
 
 (0) 
 
 
 (4) 
 4 
 
 (4) 
 4 
 
 Xhl 
 xHi 
 xhi 
 Zygotes XxHHIi (horned) 
 
 Xx Hhli ( horned ) 
 
 
 Soma XxHHIi (horned) ..., 
 
 XXhhll (hornless) 
 Xhl 
 
 XXHhll (hornless) 
 
 5 
 
 
 
 
 
 8 
 
 Gametes XHI 
 
 HJH 
 
 
 
 Soma XxHHIi (horned) 
 
 XXHHII (horned) 
 XHI 
 
 XXHHII (horned) 
 
 6 
 
 
 
 14 
 
 
 
 
 xHi 
 Zygotes XxHHIi (horned) 
 
 tor, then (designated in the table by the letter 
 I, its absence by i), will always be double in 
 the female and single in the male and, in the 
 gametes, will always be associated with the sex- 
 chromosome, which is designated through- 
 out by the symbol X; its absence by x. In 
 the zygote the single inhibitor is incapable of 
 preventing the development of the determiner 
 for the horn (JET) even when the latter is only 
 
 simplex. But the double inhibitor is capable 
 of preventing the single horn (HJi) deter- 
 miner, but not the double determiner (HH~). 
 The table gives a summary of matings 
 used, their hypothetical somatic and gametic 
 composition, and the proportion of each sort 
 of zygote that will be formed in each sex. 
 The actual frequency of offspring derived 
 from each mating is given on the left of the 
 
SCIENCE 
 
 table; the expected proportions in the more 
 complex cases being given above the actual 
 findings in parenthesis. The matings were 
 made and the offspring examined in major 
 part at the New Hampshire Agricultural Ex- 
 periment Station and in minor part at the 
 Station for Experimental Evolution. The 
 latter station was able to contribute especially 
 to the results of later generations. For 
 horned females, Dorsets were used ; for horned 
 males Rambouillets, Dorsets and the Scottish 
 4-horned race. As hornless races the Downs 
 were chiefly employed. It is not our purpose 
 now to give complete details, as the experi- 
 ments are being continued and full data will 
 be deferred until the publication of our final 
 report. 
 
 The results of the table accord very closely 
 with expectation, so that we are justified in 
 concluding that an explanation of the results 
 like that we offer is the correct one. By our 
 
 formula, then, the case of inheritance of 
 horns in sheep is brought quite into line with 
 that of other sex-limited characters, its pecul- 
 iarities being due to an inhibitor of horn de- 
 velopment that is carried in the sex-chromo- 
 some. 
 
 LITERATURE CITED 
 
 Bateson, W., 1909, "Mendel's Principles of He- 
 redity," Cambridge, Eng., University Press. 
 
 Castle, W. E., 1911, "Heredity in Belation to Evo- 
 lution and Animal Breeding," New York, Ap- 
 pleton. 
 
 Guyer, M. F., 1910, "Accessory Chromosomes in 
 Man," Biol. Bull, XIX., 219-234, PI. I. 
 
 Wood, T. B., 1905, "Note on the Inheritance of 
 Horns and Face Color in Sheep," Jour. Agrio. 
 Sri., I., 364, 365, PI. IV. 
 
 T. E. ARKELL, 
 C. B. DAVENPORT 
 DURHAM, N. H., 
 COLD SPRING HARBOR, N. Y., 
 January 29, 1912 
 
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