Summary Horns are paired appendages on the head of bovine species, comprising an inner bony core and outer keratin sheath. The horn bud forms during early fetal development but ossification of the developing horn does not occur until approximately 1 month after birth. Little is known about the genetic pathways that lead to horn growth. Hornless, or polled, animals are found in all domestic bovids. Histological studies of bovine fetuses have shown that the horn bud does not form in polled individuals. There are currently four known genetic variants for polledness in cattle on BTA1. All of the variants are intergenic, but probably affect regulation of nearby genes or long non‐coding RNAs. Transcriptomic studies suggest that the expression of two nearby long non‐coding RNAs are affected by the Celtic POLLED variant, but further studies are required to confirm these data. Candidate genes located elsewhere in the genome are involved in regulating bone formation and epithelial‐to‐mesenchymal transition. Expression of one of these candidate genes, RXFP2, appears to be reduced in the fetal horn bud of polled animals carrying the Celtic variant compared with horned individuals. Investigating horn ontogenesis and the genetic pathway by which the POLLED variants prevent horn development has implications for cattle breeding. If the genetic basis of horn bud formation and polledness is better understood, then new targets may be identified for precision genome editing to create polled individuals.

Horns are a form of headgear carried by Bovidae, including cattle, sheep, goats, and yak, and have welfare and economic implications for production systems (Simon et al., 2022). Hornless (polled) animals occur naturally in some domesticated Bovidae and are usually preferentially selected. Four dominant genetic variants are known to be associated with polled in cattle and are located within a 300-kb region on chromosome 1 (BTA1) 1 (Aldersey et al., 2020). The Celtic variant (P C ) consists of a 212-bp duplication and a 10-bp deletion (Medugorac et al., 2012). The Friesian variant (P F ) is an ~80-kb tandem duplication (Medugorac et al., 2012). The Mongolian variant (P M ) is a complex 219-bp duplication insertion with an additional 6-bp deletion and 7-bp insertion