(after Appendix 2 in Clutton-Brock & Pemberton, 2004)
Horn development and conformation is controlled by the Ho locus. It appears that there are three alleles which, in various pairings, determine the size and shape of a sheep's horns. The following two tables summarize our current understanding of what these genes do. You'll notice that each gene is written in two alternative typographic conventions: one with superscripts and one without. The first ("Formal") is what is found in printed materials, books and articles. The second ("Simple") is more convenient for use in databases, where the use of superscripts is impractical.
| Allele symbol | Description | |
| Formal | Simple | |
| Ho+ | Ho | Wild allele, dominant, horns in both rams and ewes |
| HoL | hl | Sex-limited polling allele, recessive, scurring in ewes, normal horns in rams |
| HoP | hp | Polling allele, recessive to Ho+, dominant to HoL, scurring in rams, polling in ewes. |
| Genotype | Phenotype | ||
| Formal | Simple | Rams | Ewes |
| Ho+/Ho+ | Ho/Ho | Horns | Horns |
| Ho+/HoL | Ho/hl | Horns | Horns |
| Ho+/HoP | Ho/hp | Horns | Scurs |
| HoL/HoL | hl/hl | Horns | Scurs |
| HoL/HoP | hl/hp | Horns | Polled |
| HoP/HoP | hp/hp | Scurs | Polled |
| Phenotype | Genotypes | |
| Rams | Ewes | |
| Horned | Ho+/– HoL/– |
Ho+/Ho+ Ho+/HoL |
| Scurred | HoP/HoP | HoL/HoL Ho+/HoP |
| Polled | HoP/HoP HoL/HoP |
|