Categories: Pigmentation phene

Links to possible relevant human trait(s) and/or gene(s) in OMIM: 172800 (trait) , 164920 (gene)

Mendelian trait/disorder: yes

Mode of inheritance: Autosomal co-dominant

Disease-related: no

Key variant known: yes

Year key variant first reported: 1998

Species-specific symbol: I^p

Species-specific description: This trait is characterised by "white patches and fully colored patches separated by sharp borders . . . Patch animals were white but with fully wild-colored patches, that is, both skin and all hairs were colored; the proportion of colored coat was variable and covered between 10% and 90% of the body" (Marklund et al., 1998)

History: This trait was first reported by Johansson et al. (1992) in the Swedish gene-mapping reference pedigree, i.e. in the F2 offspring from crosses between European wild boars and Swedish Yorkshire sows. In these offspring "There was a varying expression of patch, ranging from almost fully colored individuals with only a white belly to almost completely white animals with only a small patch of wild-colored coat." (Johansson et al., 1992)

Inheritance: Segregation analysis in the Swedish gene-mapping reference pedigree suggested to Johansson et al. (1992) "that patch is controlled by a variant allele [at the dominant white locus], tentatively denoted I^p, which is recessive to I [dominant white] but dominant to i [wild type]." The results of Rubin et al. (2012) indicate four alleles at this locus, namely Wild-type, Dominant white (OMIA 000209-9825), Patch (this entry) and Belt (OMIA 001745-9825). (with thanks to Leif Andersson)

Mapping: Using RFLPs for two comparative (from mouse) candidate genes, namely ALB and PDGFRA, Johansson et al. (1992) reported that the dominant white/patch locus is very tightly linked to both ALB and PDGFRA (theta=0), which are on chromosome SSC8. A third comparative candidate gene, closely linked to ALB and PDGFRA in mice, could not be mapped in pig because no RFLP could be detected. However, given its close linkage to ALB and PDGFRA in mice and in humans, KIT was also a very strong comparative positional candidate gene for porcine dominant white/patch.

Molecular basis: Marklund et al. (1998) showed that, like the dominant white allele, the patch allele is a duplication of KIT. The difference between the two alleles is that the dominant white allele also has a "G to A substitution in the first nucleotide of intron 17 disrupting the highly conserved GT pair at the 5′ splice site" resulting in the skipping of exon 17, whereas the patch allele does not have this substitution. From the results of genome sequencing of tens of pigs, Rubin et al. (2012) confirmed that the patch allele comprises a 450kb duplication that includes KIT (roughly in the middle).

Associated gene: