Recombination events restored the functional horned haplotypes in the offspring of polled parents
Breeding of genetically polled animals is a desirable approach in modern cattle husbandry for welfare and economic reasons. At least four different genetic variants associated with polledness in cattle have been identified, suggesting genetic heterogeneity. These dominant variants are located on chromosome 1 between approximately 2.42 to 2.73 Mb (reference: ARS-UCD1.3), also called the POLLED locus. Among these variants, Friesian (PF, ~80 kbp duplication) and Celtic (PC, 212 bp complex InDel) are the most observed in the majority of breeds in the production systems globally, such as in Holstein-Friesian (HF) and Fleckvieh (FV). While studies have provided strong evidence supporting the association of the PF allele with the polledness condition, it has not yet been functionally validated, unlike the PC allele.  Here, we conduct whole-genome sequencing analyses of two trios exhibiting unexpected inheritance patterns related to the PC and PF variants. In both instances, horned offspring were produced from mating pairs where one parent was homozygous for the polled variant and the other was homozygous for the ancestral horned variant. By analyzing the WGS data generated using Nanopore technology, we show that the de novo generation of the ancestral horned phenotype in both offspring was the result of distinct recombination events. Specifically, in the HF trio, it was the result of non-allelic homologous recombination in the gametes of the sire (PF/PF), while in the FV trio, it was the result of allelic homologous recombination in the gametes of the dam (PC/PF). The findings from the HF trio support the hypothesis that ~80-kbp duplication is the genetic variant responsible for the polled phenotype of Friesian origin.