24Dominant white phenotype in pigs is considered to be caused by two structural 25 mutations in KIT gene, including a 450-kb duplication encompassing the entire KIT 26 gene, and a splice mutation (G > A) at the first base in intron 17, which leads to the 27 deletion of exon 17 in mature KIT mRNA, and the production of KIT protein lacking a 28 critical catalytic domain of kinase. However, this speculation has not yet been validated 29 by functional studies. Here, by using CRISPR/Cas9 technology, we created two mouse 30 models mimicing the structural mutations of KIT gene in dominant white pigs, 31 including the splice mutation mouse model KIT D17/+ with exon 17 of one allele of KIT 32 gene deleted, and duplication mutation mouse model KIT Dup/+ with one allele of KIT 33 gene coding sequence (CDS) duplicated. We found that each mutation individually can 34 not cause dominant white phenotype. Splice mutation homozygote is lethal and 35 heterozygous mice present piebald coat. Inconsistent with previous speculation, we 36 found KIT gene duplication mutation did not confer the patched phenotype, and had no 37 obvious impact on coat color. Interestingly, combination of these two mutations lead to 38 dominant white phenotype. Further molecular analysis revealed that combination of 39 these two structural mutations could inhibit the kinase activity of the KIT protein, thus 40 reduce the phosphorylation level of PI3K and MAPK pathway associated proteins, 41 which may be related to the observed impaired migration of melanoblasts during 42 embryonic development, and eventually lead to dominant white phenotype. Our study 43 provides a further insight into the underlying genetic mechanisms of porcine dominant 44 white coat colour.45Author summary 46 KIT plays a critical role in control of coat colour in mammals. Two mutation 47 coexistence in KIT are considered to be the cause of the Dominant white phenotype in 48 pigs. One mutation is a 450-kb large duplication encompassing the entire KIT gene, 49 another mutation is a splice mutation causing the skipping of KIT exon 17. The 50 mechanism of these two mutations of KIT on coat color formation has not yet been 51 validated. In this study, by using genome edited mouse models, we found each 52 structural mutation individual does not lead dominant white phenotype, but 53 combination of these two mutations could lead to a nearly complete white coat colour 54 similar to pig dominant white phenotype, possibly due to the inhibition of the kinase 55 activity of the KIT protein, thus its signalling function on PI3K and MAPK pathways, 56 leading to impaired migration of melanoblasts during embryonic development, and 57 eventually lead to dominant white phenotype. Our study provides a further insight into 58 the underlying genetic mechanisms of porcine dominant white coat colour. 59 60 Introduction 61 Due to domestication and long term selection, dominant white is a widespread coat 62 color among domestic pig breeds, such as Landrace and Large White [1]. The dominant 63 white phen...