Color Dilution in Charolais, Highland, Simmental, Galloway, and Hereford Cattle

Dilute coat colors in Charolais, Highland, Galloway, Simmental, and American Hereford cattle are produced by variants in the premelanosome protein 17 (PMEL17) gene (also known as SILV). To date, two variants have been identified and associated with the dilute color phenotype in these breeds.

Gutiérrez-Gil et al (2007) identified a single nucleotide change c.64G>A associated with the dilution phenotype in the Charolais breed of cattle. This variant causes an amino acid change (p.Gly22Arg) and is thought to impact protein function. This variant is represented in our reports as Dc for dilute Charolais. All Charolais cattle possess the Dc variant, which explains their uniformly pale/white coat color.

Hecht (2006) identified a deletion of 3 bases (delTTC) in exon 1 of PMEL17 (summarized by Jolly et al 2008) which results in loss of a leucine amino acid in the PMEL protein sequence and is associated with the dilute phenotype in some animals of the Highland, Simmental, Galloway and Hereford breeds.  This variant is also hypothesized to impact protein function. For simplicity, the PMEL17-delTTC variant is represented here and in our reports as Dh (for dilute Highland, although it is found in several other breeds). The effect of this variant is to lighten color of red- and black-pigmented hairs. The extent of color dilution depends on the dosage of the Dh variant (one copy or two) and on the genetic make-up of the MC1R gene that controls black/red pigment production.

Color dilution caused by the Dc and Dh variants is inherited as an incompletely dominant trait. The dilute hair color of heterozygous animals (Dh/N or Dc/N) is darker than that of homozygous animals or compound heterozygotes (Dh/Dc). On a red base color (MC1R genotypes e/e or E⁺/e), one copy of either PMEL17 variant (Dh/N or Dc/N) produces a lighter red or yellow shade of color while two copies (Dh/Dh, Dc/Dc) result in a pale cream/white color. On a black base color (MC1R genotypes E^D/E^D, E^D/E⁺, E^D/e), one copy of Dh dilutes black pigment to a brown/chocolate shade called Dun in Highland and Galloway breeds. Two copies of Dh dilutes black pigment to a pale color known as Silver Dun.

Color dilution variants in the PMEL17 gene are associated with “rat-tail syndrome” (RTS), a form of congenital hypotrichosis characterized by misshaped, curly and sparse dilute colored hair, and by missing hairs at the tail switch. RTS-affected animals have health and growth performance impairments. The RTS phenotype is typically expressed in cattle that have the dominant black MC1R-E^D allele and one copy of a PMEL17 color dilution variant. Expression of RTS defect is variable. There is some evidence that supports another gene not yet identified but near PMEL17 may be critical for expression of the RTS phenotype.

A DNA test is available to determine if the PMEL17 variants are present in the genetic makeup of an animal that has a diluted or lighter than typical color phenotype. This test is appropriate for any breed or crossbred cattle that has Charolais, Highland, Galloway, Simmental or American Hereford content.