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Mapping the Horns (Ho) Locus in Sheep: A Further Locus Controlling Horn Development in Domestic Animals G. W. Montgomery, H. M. Henry, K. G. Dodds, A. E. Beattie, T. Wuliji, and A. M. Crawford The presence or absence of horns in Merino sheep is under the genetic control of the autosomal Horns (Ho) locus. Sheep chromosome OOV1 is a candidate region for the Ho locus because it shows conserved synteny with cattle chromosome BBO1 where the cattle polled locus has been located. We demonstrate that the Ho locus in sheep is excluded from sheep chromosome OOV1 and we identified linkage between the Ho locus and markers from sheep chromosome OOV10. These data suggest that there are at least two loci affecting the presence or absence of horns in sheep and cattle. The orthologous regions to OOV10 are likely to be on cattle, human, and mouse chromosomes BBO12, HSA13, and MMU14. From the AgResearch Molecular Biology Unit, Department of Biochemistry and Centre for Gene Research, University of Otago, Dunedin, New Zealand (Montgomery, Henry, and Crawford), and AgResearch, Invernay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand (Dodds, Beattie, and Wuliji). This program was supported by the Foundation for Research Science and Technology through grants to AgResearch and a grant from the New Zealand Lotteries Board. Journal of Heredity 1996;87:358-363; 0022-1503/96/S5.00 358 Genes that regulate the development of horns in bovid species show a number of interesting features. The expression of horns in some sheep strains has pleiotropic effects on wool fibers resulting in a highly medullated hairlike fleece (Dry 1955). Alleles at the same locus control sex-limited expression of horns or development of horns in both sexes (Dolling 1970). A dominant autosomal polled locus that suppresses horn formation in goats of both sexes is associated with sex reversal and abnormal sexual development (Hamerton et al. 1969; Pailhoux et al. 1994). Polled sheep are those that completely lack horns and emerged in Britain during the middle ages (Ryder 1983). Many breeds are now polled, although a number of breeds have retained horns, including the Merino breed from Spain. Rams from this breed normally have large coiled horns (Figure 1). In crosses between horned and polled Merino sheep, the phenotypes range from fully developed, large, curly horns, firmly attached to the frontal bone to polled or hornless animals with no horn development and concave depressions in the skull bone at the site (Dolling 1970). Sheep with intermediate phenotypes have short knobs or scurs that may or may not be attached to the frontal bone. The presence of horns is controlled by a single autosomal locus [Horns (Ho); COGNOSAG 1989] with three alleles (Dolling 1970). The three alleles at the Ho locus are //op, «b% and HoM (Dolling 1970). Polledness is produced by the allele Hor, which is incompletely dominant to the other two alleles in rams and almost completely dominant in ewes. Of the other two alleles, Ho* produces horns in both sexes and HoM produces sex-limited horns. The growth of horns in rams can be modified by a further autosomal locus known as Scurs (Sc; COGNOSAG 1989). The alleles interact with the Ho locus and result in the growth of scurs or aberrant horns in rams. A second locus is also known to produce horns in sheep (Dry 1955). Unlike the Ho locus, the Halo hair (HH1) locus (COGNOSAG 1989) has a marked effect on fleece characteristics. Sheep carrying the HH1N allele produce a high abundance of halo hair on the backs of young lambs. The adult mature fleece is highly medullated and is used as a specialty carpet wool. Breeding experiments have failed to separate the presence of horns from the effects on the fleece (Dry 1955), and the two effects are considered to be pleiotropic. In Bos taunts cattle, the presence or absence of horns is controlled by an autosomal locus (polled) with two alleles (Georges et al. 1993). A second locus, african horn, has been postulated in B. indicus breeds. It is epistatic to the polled locus and also has two alleles (Georges et al. 1993). Expression of the african horn locus is sex limited. A scurred locus is also present in cattle and this affects the expression of both the polled and african horn loci, with differential expression in males and females. Recently the polled locus in B. taurus Figure 1. Expression of horns in (left) Merino ram, (center) Gl Merino X Romney ram, and (right) polled New Zealand Romney ewe cattle was mapped to markers (GMPOLL-1 and GMPOLL-2) from cattle chromosome BBO1 (Georges et al. 1993). There is extensive conservation of banding patterns between cattle and sheep chromosomes (Hayes et al. 1991; Hediger et al. 1991), and genetic linkage maps for cattle (Barendse et al. 1994; Bishop et al. 1994) and sheep (Crawford et al. 1995) have recently been published. The region of the sheep genome with conserved synteny to cattle chromosome BBO1 is sheep chromosome OOVlq. We have crossed horned Merino rams with polled New Zealand Romney ewes. The Gl crossbred rams were then backcrossed to Merino ewes to produce large half-sib families. Growth and development of horns is segregating in this flock and we examined genetic markers from sheep chromosome OOV1 to test whether the Ho locus in Merino sheep is the same as the polled locus in cattle. There was no evidence of genetic linkage to markers from this region, including one of the markers linked to the polled locus in cattle. We screened markers from other chromosomes and identified linkage between the Ho locus and markers from sheep chromosome OOV10. Methods Experiments were performed in accordance with the 1987 Animal Protection (Codes of Ethical Conduct) Regulations of New Zealand after approval was granted by the Invermay Agricultural Centre Animal Ethics Committee. Sheep Flocks for Linkage Analysis All animals were run outdoors on pasture at Tara Hills High Country Research Station, Omarama. Horned Merino rams were mated with Romney ewes to generate Gl Merino X Romney rams for studies on the inheritance of wool characteristics. Four Gl rams from different sires were mated with Merino ewes and generated 183 backcross progeny born in 1992. Male lambs were castrated at birth so that male and female animals could be run in the same flock to minimize environmental effects on wool growth. The 86 ewe and 97 wether Merino X Romney hoggets were individually examined at 12 months and 15 months of age. Phenotypes at the Ho locus were recorded as depressions, bone knobs, scurs, or aberrant horns and assigned scores based on the individual's sex (Dolling 1970). Genotypes (//ohl///ohl and //ohl///op) for ewe hoggets were assigned from the phenotype scores (Dolling 1970). Genotyping of Microsatellite Markers Microsatellite markers were analyzed as described previously (Montgomery et al. 1993). Each PCR reaction contained the following constituents in a total volume of 10 ml: approximately 100 ng sheep DNA, 45 mM Tris-HCl pH 8.8, 11 mM (NH4)2SO4, 4.5 mM MgCl2, 6.7 mM B2-mercaptoethanol, 4.5 mM EDTA, 0.25 mM spermadine, 200 mg/ml BSA, 20 nM dNTPs, 400 nM CA strand primer (unlabeled), and 20 nM GT strand primer (5' end labeled with [732P]ATP using T4 polynucleotide kinase). Incubation conditions for the amplification reaction were as follows: seven cycles of denaturation at 94°C for 30 s, and a combined annealing and extension step of 63°C for 1 min (or stated annealing temperature, Table 2), followed by 20 cycles of denaturation at 90°C for 30 s, annealing and extension at 63°C for 1 min. The labeled PCR products were separated on denaturing sequencing gels and bands detected by autoradiography. Data Analysis For each family, progeny were genotyped at a test locus, only if the sire was hetero- zygous at that locus. All genotypes were scored independently by two researchers and genotypes were checked for consistency with the pedigree records. The results were then compared and any differences were identified and resolved. Twopoint LOD scores for linkage with the Ho locus were calculated using maximum likelihood methods for half-sib pedigrees (Dodds et al. 1993). All other linkage analyses were performed with CR1-MAP (Lander and Green 1987). Data on linkage between markers in this backcross flock were compared with a previous map obtained from the analysis of these markers in three-generation sheep pedigrees (IMF; Crawford et al. 1995), by calculating likelihoods for a locus order defined by the IMF map, first with distances unspecified and second with distances set to those for the IMF map. Twice the difference in lognormal likelihoods for the two analyses was compared with a chi-square distribution with degrees of freedom equal to the number of intervals in the map. Multipoint LOD scores (Ott 1991) were calculated by fixing the Ho locus at various positions along the map (with distances between markers set to those of the IMF map), but no closer than 0.01 cM to a mapped marker. Results Segregation of the Ho Locus in Merino x Romney Sheep In the Tara Hills Merino flock the rams develop large coiled horns. Ewes develop knobs or scurs, but never full expression of horns. Consequently, the main allele at the Ho locus in this flock is the Hohl allele resulting in expression of sex-limited horns. Scurs are never seen on entire males and it is unlikely that the analysis of segregation at the Ho locus would be complicated by the Sc locus. Montgomery et al • Mapping the Horns [Ho) Locus in Sheep 3 5 9 Table 1. Segregation at the Horns (Ho) locus in Merino x Romney backcross female sheep Table 2. Description of microsatellite markers from sheep chromosomes 1 and 10 Locus name Chromosome location No. of alleles Annealing temperature Source Reference BUI OarAE57 RM65 BM6438 TGLA49 AGLA17 KAP8 MAF64 1LSTS004 INRA11 BM6506 MAF109 BM864 MAF4 BM1824 OarCP38 OarDB3 TGLA441 OarHH41 AGLA226 OarVH58 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 10 10 10 10 10 10 6 5 4 4 8 1 7 7 6 7 6 8 5 8 4 6 7 8 7 6 3 57 63 55 63 Tf> TD 50 64 50 55 TD 58 55 65 63 55 63 TD 63 TD 63 Bovine Ovine Bovine Bovine Bovine Bovine Ovine Ovine Bovine Bovine Bovine Ovine Ovine Ovine Bovine Ovine Ovine Bovine Ovine Bovine Ovine Bishop et al. 1994 Penty et al. 1993 Bishop et al. 1994 Bishop et al. 1994 Georges and Massey 1992 Georges and Massey 1992 Wood et al. 1992 Swarbrick et al. 1991 Bishop et al. 1994 Bishop et al. 1994 Bishop et al. 1994 Swarbrick and Crawford 1992 Bishop et al. 1994 Buchanan et al 1991 Bishop et al. 1994 Ede et al. 1995 Bancroft 1993 Georges and Massey 1992 Henry et al. 1993 Georges and Massey 1992 Pierson et al. 1993 Horns genotype Family Hd«IH<f Ho»'/Ho» 4268 4269 4270 4271 14 8 9 12 10 16 7 10 The heads of each female offspring were examined. Animals with a depression or a bone knob in a depression were assigned an Ho^'/Hcf genotype (Dolling 1970). Animals with a bone knob or scur and no depression were assigned as Hox'/Ho1" genotype (Dolling 1970). All of the Merino sires used to generate the Gl rams had large well-developed horns. The four Gl rams grew horns, but these failed to develop fully into the large curly horns characteristic of the Merino breed and were classified as a large scur or aberrant horn (Dolling 1970; Figure 1), indicating that the Gl rams were heterozygous at the Ho locus. Examination of the progeny of the four Gl rams demonstrated that most male progeny developed scurs. Homozygous HoM/HoM males did not develop full horns because they were castrated at birth. Consequently, it was not possible to differentiate between Hohl/Hoh> and HoM/Ho? male progeny. Two of 97 male progeny were recorded as polled (Hop/Hor), indicating that there is a low frequency of the Hop allele in the Merino dams. The presence of the Hop allele in the dams could introduce errors in the assignment of phenotypes at the Ho locus. These errors would be expected to reduce the significance of any linkage detected, but the low frequency of polled males observed in the progeny of these matings suggests that the effect of these errors would be small. Male progeny were excluded from subsequent linkage analysis. Female progeny were completely polled or developed small scurs. The phenotypes of the females closely followed the descriptions for crosses between horned and polled Merino sheep (Dolling 1970). All four Gl sires had both polled and scurred female progeny (Table 1), demonstrating that all were heterozygous for Hohl/Hop alleles. Distributions of polled and scurred progeny in the four families were consistent with Mendelian segregation of the Ho* allele; across all families there were 43 polled progeny (HoMIHop) and 43 progeny (//o/i////ohl) that developed scurs. Exclusion of the Ho Locus From Sheep Chromosome OOVI Markers from sheep chromosome OOV1 (Table 2) were analyzed in the Merino 3 6 0 The Journal of Heredity 1996.87(5) - TD = touchdown PCR procedure: 95°C, 45 sec; 60°C, 1 min; x 3 cycles/95°C, 45 sec; 57°C, 1 min; x 3 cycles/95°C, 45 sec; 54°C, 1 min; x 3 cycles/95°C, 45 sec; 51°C, 1 min; x 3 cycles/95°C, 45 sec; 48°C, 1 min; x 20 cycles. backcross pedigrees. There are 10 markers in common on the sheep and cattle linkage maps (Figure 2). The markers linked to the polled locus in cattle are AGLA17 (GMPOLL-2) and TGLA49 (GMPOLL1; Georges M, personal communication). All the markers were informative in at least one of the families with the exception of AGLA17. AGLA17 was monomorphic in sheep and could not be placed on the map. Analysis of cosegregation of the Ho phenotype and individual markers, including TGLA49, showed no evidence of linkage. A multipoint linkage analysis with all 14 markers demonstrated that the Ho locus was excluded from the region of sheep chromosome OOV1 covered by this linkage group with a maximum LOD score -4.7 across the linkage group. The maximum LOD score around the likely location of the cattle polled locus, between markers BM6348 and ILSTS004 (Figure 2), was -43.0. Location of the Ho Locus in Sheep A further 34 markers from across the sheep genome were screened in the Merino backcross pedigrees. Linkage to the Ho phenotype was first detected with the microsatellite marker OarVH58. Three of the four sires were heterozygous for alleles at the OarVH58 locus and there were no recombinants between OarVH58 and the Ho phenotype (Table 3). For marker OarVH58, the allele segregating with the Hcf allele was the same in three of the families. However, these alleles were not fixed in the parental breeds and estimates of the allele frequencies at the OarVH58 locus in 40 unrelated Merino and 40 unrelated Romney sheep demonstrated that the OarVH58 allele associated with the Hop allele from Romney sheep was present in the Merino breed at a frequency of 0.51. The OarVH58 allele associated with the Hohl allele from Merino sheep was present in the Romney breed at a frequency of 0.27. The microsatellite OarVH58 belongs to a linkage group from sheep chromosome OOV10 (Figure 3; Crawford et al. 1995). Six markers from this linkage group were analyzed in the flock (Table 2). The Ho.phenotype showed significant linkage to two additional markers from this linkage group (Table 3). OarHH41 was linked to the Ho phenotype with a maximum LOD score of 10.5 at a distance of 8 cM. AGLA226 was linked to the Ho phenotype with a maximum LOD score of 5.3 with no recombinants. Data on linkage between the markers on chromosome OOV10 were compared with the analysis of these markers in three-generation sheep pedigrees (Crawford et al. 1995). There were no differences between the maps generated from these and the IMF pedigrees. The position of the Ho locus was estimated to be close to AGLA226 (Figure 4). The one LOD support interval (region where the LOD was within one of the maximum LOD) for the location of the Ho locus extended from 63 cM to beyond OarVH58 (Figure 4). Discussion Analysis of a sheep backcross population from the Merino x Romney sires shows Sheep Cattle Chromosome 1 & 3 (Bishop et al 1994) Chromosome 1 3.3 cM 9.4 cM 19.6 cM 1.2 cM 3.0 cM BL41 i— B L 4 1 OarAE57 - 0 E 5.0 cM BM6438 TGLA49 KAP8 - MAF64 12.4 cM - ILSTS004 FCGR2 RM065 RM065 12.9 cM Chromosome 1 (Barendse et al 1994) ,SOD1 'TGLA49 BM6438 6.4 cM AGLA17 20cM — RM095 23.5 cM RM095 29 cM 10.2 cM 36.7 cM 5.3 cM ILSTS004 BM4307 21 cM 17.4 cM - INRA11 ~ OarDB6 -CRYG8 BM6506 -BM6506 BR2724 BM864 10.2 cM - URB038 TGLA130 MAF109 CSSM32 BM864 30.7 cM - CSSM32 9.8 cM 12.0 cM - 26 cM 18.0 cM -TGLA415 6.8 CM 17 cM 13.5 cM 17.4 cM 2.8 CM -UMPS INRA11 BM1312 9.1 cM 8.5 cM - TGLA57 6.9 cM 8.6 cM 6.5 cM _ MAF4 -BM1824 -TF 24 cM BL28 - CSSM19 BM1824 TF MAF46 10 cM MAF46 CRYA1 12 cM -GMBT7 20.3 CM BM3205 11 cM — CSRD1613 Figure 2. A comparison of the linkage maps from sheep chromosome 1 (Crawford et al. 1995) with the published maps for cattle chromosome 1 and a portion of cattle chromosome 3 close to the centromere (Barendse et al. 1994; Bishop et al. 1994). Markers in common between the sheep and cattle chromosomes are shown in bold type and markers in common between the published cattle maps are shown in italics. segregation for the Ho locus in the female progeny. We have demonstrated that this locus is not linked to markers from sheep chromosome OOV1, including the marker TGLA49, linked to the polled locus in B. taurus cattle (Georges et al. 1993). The Ho locus was linked to the markers OarHH41, OarVH58, and AGLA226 on sheep chromosome OOV10. The orientation of the linkage and physical maps for sheep chromosome 10 have not been defined, but linkage between OarHH41 and RP11 suggests that the Ho locus maps toward the telomere of this chromosome (Crawford et al. 1994). One explanation for these results is that the polled condition in sheep and B. taurus cattle results from a mutation in the same gene, but that this locus is located on chromosome BBO1 in cattle and chromosome OOV10 in sheep. Sheep chromosome OOV1 is a metacentric chromosome formed from a Robertsonian fusion of ancestral chromosomes making up chromosomes BBO1 and BBO3 in the cattle karyotype (Hediger et al. 1991). The polled gene in cattle maps approximately 17 cM from TGLA49. If polled is proximal to TGLA49 it could lie close to the centromere (Georges et al. 1993). It is possible that an additional translocation event during evolution of the ovine metacentric chromosome OOV1 could have relocated the orthologous locus in sheep to OOV10. Comparison of the linkage maps of sheep and cattle (Bishop et al. 1994; Crawford et al. 1995) reveals at least one chromosomal rearrangement between the species. An alternative explanation is that loci Montgomery et al • Mapping the Horns (Ho) Locus in Sheep 3 6 1 mal female (Hamerton et al. 1969). Histologically the gonads of intersex animals were testicular in appearance, but germ cells were never seen in the seminiferous tubules after birth (Hamerton et al. 1969). Y-specific sequences including SRY (sexdetermining region Y chromosome) were not detected in 60.XX pseudohermaphrodite goats (Pailhoux et al. 1994). The polled genes in sheep and cattle are not associated with abnormal sexual development. The "polled" locus in goats maps to the distal end of goat chromosome CHU (Vaiman et al. 1996) and appears to be a different gene than the polled locus in cattle and the Horns locus in sheep. The linkage maps for OOV10 and BBO12 have few markers in common (Bishop et al. 1994; Crawford et al. 1995). Erythrocyte antigen B (EAB) has been mapped to OOV10 (Crawford et al. 1995) and BBO12 (Bishop et al. 1994). Coagulation factor X (F10) has been mapped to OOV10q33-qter by in situ hybridization (Pearce et al. 1994) and to BBO12 by somatic cell hybrid analysis (Fries et al. 1993). The retinoblastoma gene (RBI) has been mapped to sheep chromosome OOV10, to cattle chromosome BBO12, and goat chromosome 12 (Hayes et al. 1993). The orthologous genes for RBI have been mapped to human chromosome HSA13 and mouse chromosome MMU14. Table 3. Pairwise linkage data to the Horns (Ho) phenotype Recombination fraction Locus N 0.01 0.05 0.10 0.15 0.20 0.30 0.40 2^ e OarVH58 AGLA226 OarHH41 TGLA441 0arDB3 OarCP38 60 21 68 60 39 60 10.86 5.21 7.84 -29.82 -15.87 -26.62 10.16 4.86 10.27 -14.36 -7.76 -13.69 9.24 4.41 10.38 -8.24 -4.51 -8.42 8.26 3.93 9.79 -5.04 -2.78 -5.54 7.21 3.43 8.88 -3.05 -1.66 -3.67 4.89 2.34 6.46 -0.90 -0.46 -142 2.23 1.11 3.25 -0.12 -0.59 -0.33 11.03 5.30 10.49 0.00 0.00 0.00 0.00 0.00 0.08 0.50 0.50 0.50 Lod scores for linkage were calculated at different recombination fractions using maximum likelihood methods for half-sib pedigrees (Dodds et al. 1993). The number of progeny scored for each comparison is given by N. mapped to BBO1 and OOV10 are not orthologous genes. There are at least two loci affecting the presence or absence of horns in sheep. In addition to the Ho locus studied in these pedigrees, the HH1 locus causes the development of horns and has marked pleiotropic effects on fleece characteristics (Dry 1955). The Ho locus is not associated with any known effects on the fleece and is thought to be a different locus (Dolling 1970; Dry 1955). In B. indicus cattle, the african horn locus is epistatic to the polled locus with sex-limited expression (Georges et al. 1993). Since the expression of the major allele at the Ho locus in sheep shows sex-limited expression, and the cattle polled locus and sheep Ho locus map to different chromosomes, the sheep Ho locus may be orthologous to the african horn locus. The cattle homologue of sheep chromosome OOV10 is BBO12 (Hediger et al. 1991) and the african horn locus may be located on this chromosome. Similarly the HH1 locus in sheep may be orthologous to the polled locus in cattle and sheep chromosome OOV1 is a potential candidate region for this gene. The marker TGLA49 is located close to a cluster of keratin genes on chromosome OOV1 (Crawford et al. 1995; Wood et al. 1992). The relationship of the polled genes in sheep and cattle to those in other ungulates is not known. In goats, the autosomal dominant polled locus has complete penetrance in both sexes and is associated with abnormal sexual development (Hamerton et al. 1969). Genetic females (60,XX) that were homozygous for the polled (PP) locus demonstrate a range of phenotypes from almost normal male to almost nor- OarCP38 7.1 CM 20-1 OarDB3 5.3 CM TGLA441 15.6CM 10- - EAB O O 0- 27.2 CM _ OarHH41 -10-lt I o\ 18.7 CM oo — AGLA226 OarVH58 CO Q_ O 3 Figure 3. The current linkage map for sheep chromosome 10 (Crawford et al. 1995) including microsatellite markers used in the linkage study with the Ho locus. 3 6 2 The Journal of Heredity 199687(5) I , I 10 I 8 Q ra O • 1 1 1 20 30 40 i I I | M 50 3 Distance (Kosambi cM) 60 70 | (80 <5 O Figure 4. A mutilocus linkage analysis with the Horns (Ho) locus and markers from sheep chromosome 10 in female progeny of Merino x Romney backcross pedigrees. The recombination frequencies between markers were set from analysis in independent three-generation pedigrees and converted to centimorgan using Kosambi's mapping function. The nonshaded area corresponds to positions where the LOD score is within one unit of the maximum (one LOD support interval; Ott 1991). Further analyses with more animals and additional markers from this region, including orthologous genes from other species, will be required to locate the Ho locus more precisely. However, linked markers identified in this study, used in conjunction with breeding records, would provide a DNA test for carriers of alleles at the Ho locus. References Bancroft D, 1993. Genetic variation and fitness in Soay sheep (PhD dissertation). Cambridge, England: University of Cambridge. 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Montgomery GW, Crawford AM, Penty JM, Dodds KG, Ede AJ, Henry HM, Pierson CA, Lord EA, Galloway SM, Schmack AE, Sise JA, Swarbrick PA, Hanrahan V, Buchanan FC, and Hill DF, 1993. The ovine Booroola fecundity gene (FecB) is linked to markers from a region of human chromosome 4q. Nature Genet 4:410-414. Dry FW, 1955. The dominant N gene in New Zealand Romney sheep. Austr J Agric Res 6:725-769. Ott J, 1991. Analysis of human genetic linkage, rev. ed. Baltimore: John Hopkins University Press. Ede AJ, Pierson CA, and Crawford AM, 1995. Ovine microsatellites at the OarCP34, OarCP38, OarCP43, OarCP49, OarCP73 and OarCP79 loci. Anim Genet 26: 130-131. Pailhoux E, Cribu EP, Chaffaux S, Darre R, Fellous M, and Cotinot C, 1994. Molecular analysis of 60.XX pseudohermaphrodite polled goats for the presence of SRY and ZFY genes. J Reprod Fertil 100:49M96. Fries R, Eggen A, and Womack JE, 1993. The bovine genome map. Mamm Genome 4:405-428. Pearce PD, Ansari HA, Maher DW, Broad TE, Cambridge LM, Lewis PE, Burkin DJ, and Jones C, 1994. The assignment of fourteen new loci to ovine chromosomes. In: Proceedings of the Fifth Australasian Gene Mapping Workshop, Armidale. Georges M, Drinkwater R, King T, Mishra A, Moore SS, Nielsen D, Sargeant LS, Sorensen A, Steele MR, Zhao X, Womack JE, and Hetzel J, 1993. Microsatellite mapping of a gene affecting horn development in Bos taunts. Nature Genet 4:206-210. Georges M and Massey J, 1992. Polymorphic DNA markers in Bovidae. World Intellectual Property Organisation publ. no. 92/13120. Hamerton JL, Dickson JM, Pollard CE, Grieves SA, and Short RV, 1969. Genetic intersexuality in goats. J Reprod Fert Suppl 7:25-51. Penty JM, Henry HM, Ede AJ, and Crawford AM, 1993. Ovine microsatellites at the OarAE16, OarAE54, OarAE57, OarAEl 19 and OarAE129 loci. Anim Genet 24: 219. Pierson CA, Hanrahan V, Ede AJ, and Crawford AM, 1993. 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Ovine microsatellites at the OarHH35, OarHH41, OarHH44, OarHH47 and OarHH64 loci. Anim Genet 24: 222. Lander ES and Green P, 1987. Construction of multilo- Vaiman D, Koutita O, Oustry A, Elsen J-M, Manfredi E, Fellous M, and Cribiu EP, 1996. Genetic mapping of the autosomal region involved in XX sex-reversal and horn development in goats. Mamm Genome 7:133-137. Wood NJ, Phua SH, and Crawford AM, 1992. A dinucleotide repeat polymorphism at the glycine- and tyrosine-rich keratin locus in sheep. Anim Genet 23:391. Received May 17, 1995 Accepted January 19, 1996 Corresponding Editor: Stephen J. O'Brien Montgomery et al • Mapping the Horns {Ho) Locus in Sheep 3 6 3