THE mode of inheritance of the horned, scurred,

Transcription

1 The Journal of Heredity :^*00.. Inheritance of the horned, scurred, and polled condition in cattle CHARLES R. LONG AND KEITH E. GREGORY THE mode of inheritance of the horned, scurred, and polled condition in cattle has been reported by several workers. White and Ibsen 0 presented a hypothesis that included four independent loci to explain their observations and reports in the literature up to the time of their study. Their hypothesis, which is the most complex that has been proposed for the inheritance of these characteristics, included the following model: ) Pcompletely dominant gene for the polled condition, completely epistatic to horns (H) in both sexes; pthe absence of P. ) Hgene for horns; always present in both sexes in the homozygous state and epistatic to the gene for scurs (Sc). hdoes not exist in domestic cattle. As pointed out by Shrode and Lush, this locus serves only to symbolize the genetic complex that makes them cattle. ) HaAfrican horn gene epistatic to P in males; not certain in females. It is present in most breeds but is at a low frequency in many breeds. This gene does not modify the expression of horns on an otherwise horned animal. hathe absence of Ha. ) Scthe gene for scurs. The expression of this gene is sex-influenced. The heterozygote (Scsc) is usually scurred in males, but only the homozygote (ScSc) is scurred in females. White and Ibsen 0 assumed that these four genes segregated independently. Other workers have postulated that only one locus is involved in the inheritance of the horned, scurred, and polled condition. Spillman as well as Lloyd- Jones and Evvard interpreted the gene for polled (P) to be dominant to the gene for horns and many of the heterozygotes of both sexes to be scurred. Watson interpreted the gene for polled to be completely dominant in females and horns to be inhibited, but not completely suppressed, in heterozygous males; he also interpreted the level of suppression to vary among breeds. Cole reported results indicating that the polled condition is inherited as dominant to horns and that scurs occur more frequently The authors are, respectively, associate professor, Department of Animal Science, Texas A&M University, College Station, TX ; and research geneticist, U.S. Meat Animal Research Center, Science and Education Administration, USDA, Clay Center, Nebraska. This paper was prepared while the second author was in residence as a visiting professor on the faculty of the Department of Animal Science, Texas A&M University through an assignment of the Intergovernmental Personnel Act. and are larger in males than in females. The results of Gowen indicated separate loci for the inheritance of horns and scurs; he also observed that a higher frequency of scurred males than females suggested that scurs are inherited as a sex-influenced trait. Buchanan-Smith provided evidence for the presence of the African horn (Ha) gene. White and Ibsen 0 suggested that this gene has its own locus. Williams and Williams presented both a comprehensive review of the literature on the inheritance of horns and scurs and also data that they interpreted to support the inheritance model proposed by White and Ibsen l0 with the exception that Williams and Williams " interpreted their data to indicate the gene for scurs was recessive. Blackwell and Knox reported a study of the scurred condition in a herd of Angus cattle. With one exception, their observations indicated that scurs are inherited as a sex-influenced trait with the male heterozygote (Scsc) being scurred; but in the female, only the homozygote (ScSc) is scurred. The exception they noted was that a scurred male sired only one scurred male in male progeny. They suggested the sire to be genotype scsc. In a review article, Shrode and Lush suggested that the one-factor hypothesis, in which the gene for polled (P) is dominant to the gene for horns (p) in both sexes, would explain most of the reports relating to inheritance of polledness, scurs, and horns in cattle. However, they recognized that scurs occur more frequently and are larger in males than in females and that some exceptions could not be explained by a single-locus hypothesis. Materials and Methods Nonregistered Angus and Hereford dams were inseminated in and with semen from registered Angus, polled Hereford, and Hereford sires at the U.S. Meat Animal Research Center, Clay Center, Nebraska. The Hereford dams were genotypically all horned (pp) but were dehorned, and the Angus dams were all polled and none of them had scurs. Fifty-one polled, Angus sires were used in the experiment during the years. Forty-four polled Hereford and Hereford sires were used during the years; were classified as polled and, as scurred, and as horned. Angus, polled Hereford, and Hereford sires were mated to both Angus and Hereford dams. Male progeny were castrated immediately after birth. A total of 0 progeny were produced in the experiment. None of the progeny produced from these matings was dehorned, and all progeny were Downloaded from jhered.oxfordjournals.org at University of Saskatchewan on November, 00

2 The Journal of Heredity classified between 00 and 0 days of age for head condition, head shape at poll, presence of scurs, scur size (if present), and presence of bumps under hide consistent with the procedure reported by Wilson et al.. The classification procedure for each factor evaluated is as follows: head condition = polled (no growths), = scurred (loose growths), and = homed (growths firmly attached to skull); head shape at poll = flat, = round, =, and = extremely ; scur size0 = none, = scab (no horny shape), = light scur (less than inches long), = medium scur (- inches long), and = heavy scur (over inches long); and bumps = no bumps, and = bumps. Results and Discussion The frequency of horned, scurred, and polled progeny classified by type of mating are presented in Table I. The inheritance model assumed for the polled, horned, and scurred condition reflecting the basis for classification is as follows: Genotype PP ScSc pp Scsc pp SCSC Pp ScSc Pp Scsc Pp scsc pp ScSc pp Scsc pp scsc Table I. The authors also considered a single-locus model with multiple alleles to explain the inheritance of horned, scurred, and polled conditions. However, a single-locus model with multiple alleles is not adequate to explain the results. The inheritance model presented does not explain a situation reported by cattle breeders of males siring only polled progeny. Such could be the result of the absence of the African horn gene (Ha) in the female population to which such males are mated. The results observed in this study are generally consistent with the inheritance model proposed by White and Ibsen 0 of four separate loci; i.e., polled, horned, African horn, and scur, with the polled gene (P) epistatic to the gene for horns (H), the African horn gene (Ha) epistatic to the polled gene (P), at least in males, and with scurs (c) inherited as a sex-influenced trait. To explain the results of this study, it is necessary to modify the hypothesis of White and Ibsen 0 to the effect that males heterozygous for scurs (Scsc) must also be heterozygous for polled (Pp) if scurs are to be expressed. Progeny of Angus sires A single horned male was produced from a horned Hereford dam. This horned male is interpreted to be due to either the presence of the African horn gene (Ha) in the Hereford dam or that one of the Angus sires used was heterozygous for polled (Pp). This sire produced a total of progeny of both sexes from both Angus and Hereford dams. All were except two male progeny with horned Hereford dams. Because no other horned progeny were produced in matings of Angus sires Number of progeny observed to be, scurred on both sides, scurred on one side, or horned from Angus, polled Hereford, and Hereford sires mated to Angus and Hereford dams Sire class* Angus ( sires) PP scsc PP Scsc Polled Hereford ( sires) Nonscurred ( sires) PP scsc PP Scsc Nonscurred ( sires; misclassified) Pp scsc Pp Scsc ( sires) Pp Scsc or PpScSc Hereford ( sires) pp scsc or pp Sc- 0 Male progenyt Sc LR Angus dams H 0 Female progenyt Sc LR H Male progeny Sc LR Hereford dams H Female progenyt Sc LR H Downloaded from jhered.oxfordjournals.org at University of Saskatchewan on November, 00 * Genotypes listed were assigned based on proposed inheritance model t = ; Sc = scurred on both sides; LR = scurred on one side; H = homed

3 Long and Gregory: Inheritance of horns and scurs in cattle FIGURE Animals showing variation in the phenotypic expression of the polled scurred-horned complex. Aclean polled; Bscurred; and Chorned. Downloaded from jhered.oxfordjournals.org at University of Saskatchewan on November, 00 with horned Hereford dams, this horned male is unlikely to be due to an Angus sire's being heterozygous for polled (Pp). The one scurred female produced by an Angus dam and the 0 scurred females produced by Hereford dams were the progeny of Angus sires and indicate that these Angus sires were heterozygous (Scsc) for scurs. The fact that these sires were not scurred indicates that males heterozygous for scurs (Scsc) must also be heterozygous for polled (Pp) if scurs are to be expressed. Thus, we suggest that males that sire scurred females are genotype PPScsc. The fact that Angus sires produced scurred female progeny from the total of Angus sires used indicates a frequency of 0. of the gene for scurs in this population of males. The percentage of scurred males obtained from mating of the Angus sires classified PPscsc to horned Hereford dams indicates a frequency of the gene for scurs (c) of 0. in this sample of horned Hereford dams. The difference between male and female progeny in percentage showing scurs is convincing evidence that scurs are inherited as a sex-influenced trait. However, the requirement for heterozygous scurred (Scsc) males also to be heterozygous for polled (Pp) if scurs are to be expressed indicates incomplete penetrance.

4 The Journal of Heredity Progeny of polled Hereford sires Eleven of the polled Hereford sires that did not sire horned progeny sired scurred females. We suggest that these sires were genotype (PPScsc) and that scurs were not expressed because the sires were not heterozygous for polled (Pp). The polled Hereford sires that produced scurred female progeny from the total of used that did not produce horned progeny indicate a frequency of 0. of the gene for scurs in this population of polled Hereford males. The percentage of scurred males obtained from mating the non-scurred polled Hereford sires classified as Ppscsc to horned Hereford dams indicates a frequency of the gene for scurs (c) of 0. in this sample of the horned Hereford females. The results show 0 percent of the male progeny and percent of the female progeny by Angus sires classified PPScsc with Hereford dams to have scurs; whereas, percent of the male progeny and percent of the female progeny by polled Hereford sires classified PPScsc with Hereford dams were scurred. Nine of the polled Hereford sires classified as proved to be heterozygous for polled (Pp) by producing horned progeny in matings with both Angus and horned Hereford dams. One of the polled Hereford sires classified in this category produced two female progeny that were scurred. This sire was likely misclassified and was scurred with genotype PpScsc. The sires that produced horned progeny with no scurred female progeny are believed to be of genotype Ppscsc. Three horned male progeny were produced in matings with Angus dams. As indicated, the Angus dams used in this experiment were not registered purebreds. This Head shape ) Flat ) Round ) Peaked ) Extremely Table II. Sex Combinedt Combined Combined Combined Combinedt result indicates that either some of the Angus dams were heterozygous for polled (Pp) or that they were heterozygous for the gene for African horns (Haha) and that the gene for African horns (Ha) is not epistatic to the gene for polled (P) in females. These Angus dams were polled. White and Ibsen 0 did not have data relating to whether the gene for African horns (Ha) is epistatic to the gene for polled (P) in females. The four polled Hereford sires that were classified as scurred all produced horned progeny. Their genotype was probably PpScsc or PpScSc. Both Angus and horned Hereford dams produced horned progeny from these matings. A high percentage of scurred males used in the polled Hereford breed have been reported to the authors by polled Hereford breeders to sire horned progeny when mated to horned (pp) or heterozygous polled (Pp) females. These males are probably genotype PpScsc. These nonexperimental observations by polled Hereford breeders indicate further that males heterozygous for scurs (Scsc) must also be heterozygous for polled (Pp) before scurs are expressed. Progeny of horned Hereford sires The two horned Hereford sires produced horned progeny, one with an Angus dam. The horned male with an Angus dam indicates again that either some of the Angus dams were heterozygous for polled (Pp) or for the African horn gene (Ha) and that the African horn gene was not epistatic to the gene for polled (P) in females. The horned Hereford sires did not produce scurred female progeny. Thus, their genotype was likely ppscsc. Frequency of head shape classes with presence of scurs, size of scurs, and presence of bumps under hide of males, females, and sexes combined ) Absent (.J!%) (.%) 0(.%) (.%) (.%) Scurs* ) Present (.0%) (.0%) (.0%) (.0%) 00 (.%) ) Scab (0.%) 0 (.%) (.0%) (.%) (0.%) Size of scurs ) Light (.%) (.%) (.%) (.%) ) Medium (.%) (.%) (.%) (.%) ) Heavy (.%) (.%) (.%) (.%) ) Absent (.%) (.0%) 0 0 (.%) (0.0%) 0 (.%) Bumps ) Present (.%) (.%) (.%) (.%) 0 0 (.%) Downloaded from jhered.oxfordjournals.org at University of Saskatchewan on November, 00 * Small inconsistencies in total numbers occur, e.g., animals exhibited no scurs but only of these are used for bump scores; animal was dropped because of different bump scores for left and right side; similarly, only of the 00 scurred cattle had usable size scores t Percentage of flat, round,, and extremely head shapes, respectively, for each column classification as shown t Percentage of each subclass within each scur, size of scurs, and bump classification as shown

5 Long and Gregory: Inheritance of horns and scurs in cattle Classification of head shape, scurs, and bumps Frequency of head shape classes, presence and size of scurs, and presence of under-hide bumps are presented in Table II. Contingency of head shape, presence and size of scurs, and presence of under-hide bumps are presented in Table III, along with contingency chi-square values for males and females and for sexes combined. The significant (P < 0.00) chi-square value for males, females, and sexes combined shows that the presence of scurs is not independent of head shape. Scurs were observed at a significantly higher frequency in descending order on flat, round,, and extremely polls. The significant (P < 0.00) chi-square value for size of scur for males and sexes combined shows that size of scur is not independent of head shape. Scur size tended to be observed at a higher frequency in descending order from flat to extremely polls. Summary Sex Sexes combined The results of this study from mating registered Angus, polled Hereford, and Hereford sires to nonregistered Angus and horned Hereford dams are in general agreement with the previously proposed hypothesis that four independent loci control the inheritance of horns and scurs in cattle. Observations on 0 progeny produced from mating Angus, polled Hereford, and Hereford sires to Angus and horned Hereford dams can be explained by assuming that all cattle are homozygous for the gene for horns (H), which is epistatic to the gene for scurs (c). The gene for polled (P) is completely epistatic to H in both sexes. The gene for African horns (Ha) is epistatic to polled (P) in males, but these results indicate that the gene is probably not epistatic to polled in females. The gene for scurs is sex-influenced with incomplete penetrance in males with ScSc females scurred and P-ScSc and PpScsc males scurred, i.e., males that are heterozygous for scurs must also be heterozygous for polled for scurs to be expressed. One polled Hereford sire that was classified as and sired horned progeny also sired two scurred female progeny. This sire was likely misclassified in regard to scurs and his most likely genotype was PpScsc. On the basis of these results, avoidance of use of sires with scurs should result in increased frequency of the gene for polled in the polled Hereford breed. These results showed a frequency of 0. for the gene for scurs in the sample of Angus sires used. The frequency of the gene for scurs was 0. in polled Hereford sires that did not produce horned progeny. The frequency of the gene for scurs averaged 0. in the sample of horned Hereford dams that produced male progeny in this experiment. The results from this study showed a higher frequency Table III. Contingency of head shape with presence of scurs, size of scurs, and presence of bumps under hide of males, females, and sexes combined^ Head shape l)flat ) Round ) Peaked ) Extremely Chi-squarett ) Flat ) Round ) Peaked ) Extremely Chi-squareM ) Flat ) Round ) Peaked ) Extremely Chi-squareM Scurs ) Absent ) Resent.*** () () 0 () ().0*** 00.*** ) Scab () 0 () Size of scurs ) Light ) Medium ) Heavy () () 0.00**.0*** (ID () () (0) Insufficient numbers o test () () () (0) Bumps ) Absent ) Present 0 () ().. () () *** Downloaded from jhered.oxfordjournals.org at University of Saskatchewan on November, 00 t Classes from Table II were combined to provide appropriate groups for testing; parentheses around values indicate classes combined tt Contingency chi-square calculated as described by Steel and Torrie t/»<0.0 * P < 0.0 **P < 0.0 *** P < 0.00

6 00 The Journal of Heredity of scurs, scurs of larger size, and a higher frequency of bumps to be associated with shape of poll in descending order from flat, round,, and extremely. Thus, presence and size of scurs and presence of bumps are not independent of shape of poll. Selection of cattle with a poll should contribute to a reduction in the percentage of scurred animals produced. Literature Cited. BLACKWELL, R.L. and J.H. KNOX. Scurs in a herd of Aberdeen-Angus cattle. J. Hered. :-... BUCHANAN-SMITH, A.D. The inheritance of horns in cattle; some further data. J. Genet. :-... COLE, L.J. The Wisconsin experiment in cross-breeding cattle. Proc. World Dairy Congress. :-... GOWEN, J.W. Studies in inheritance of certain characters of crosses between dairy and beef breeds of cattle. J. Agric. Res. :-... LLOYD-JONES, O. and J.M. EVVARD. Inheritance of color and horns in blue-gray cattle. Iowa Agric. Exp. Sta., Ames. 0:-0a... SHRODE, R.R. and J.L. LUSH. The genetics of cattle. Adv. Genet. :0-... SPILLMAN, W.J. Mendel's law in relation to animal breeding. Proc. Am. Breeders' Assoc. : STEEL, R.G.D. and J.H. TORRIE. Principles and Procedures of Statistics. McGraw-Hill, New York. 0.. WATSON, J.A.S. A Mendelian experiment with Aberdeen- Angus and West Highland cattle. J. Genet. : WHITE, W.T. and H.L. IBSEN. Horn inheritance in Galloway-Holstein cattle crosses. J. Genet. :-... WILLIAMS, H.D. and T. WILLIAMS. The inheritance of horns and their modifications in polled Hereford cattle. J. Hered. :-... WILSON, L.L., D.D. SEAMA, and K.A. BARBER. The intrigue of Charolais polled genetics. The Charolais Way. September,. Downloaded from jhered.oxfordjournals.org at University of Saskatchewan on November, 00