Progress on genetic mapping and genetic mechanism of cattle and sheep horns.

Horns are cranial appendages, which are unique in ruminants. Cattle (Bos taurus) and sheep (Ovis aries) cranial appendages exhibit various forms of morphology, including wild-type two-horn phenotype, polled phenotype and scur phenotype. These animals provide an ideal model for studies on the underlying relationship between quality and quantitative traits of cattle and sheep horn and the molecular mechanisms of horn phenotype as a polygenic regulation for quality traits. In recent years, some research progresses of cattle and sheep horns are successively reported, which helps us better understand the evolutionary origin of new organ, the effects of natural selection, sex selection and artificial selection on horn phenotypes. In this review, we introduce in details the recent advances on the research of horn traits in cattle and sheep, and summarize the genetic mapping of multi-horned phenotypes, the genetic mapping of polled locus, and studies on scur phenotype. Moreover, we discuss potential problems in such research, thereby providing a reference for investigation on the genetic mechanisms of horn traits in ruminants.