I-Locus controls the intensity of the red pigment in dogs through affecting the expression of the pheomelanin pigment. This means that recessive I-allele is responsible for cream, cream-white or pure white coat colour.
In dogs there are two types of pigment that create coat colour: eumelanin and pheomelanin. While the eumelanin is a dark pigment causing black coat colour, the pheomelanin is a red pigment. I-locus causes so called »extreme dilution« of the pheomelanin. The mutation on I-locus does not affect the eumelanin so that the black coat and the black hair ends remain black.
Pure white coat colour in some breeds are a combination of extreme dilution of the pheomelanin (I-Locus) and the absence of eumelanin (E-locus genotype e/e). Dogs with pure white coat colour are not albino and retain pigmentation in their nose, leather, eye rims, lips, or pads.
Although an i/i genotype at I-Locus cause cream to white coat colour in many dog breeds, it is probable that other modifier alleles exist and explain the cream to white coat colour in some breeds.
Sample: EDTA whole blood (1.0 ml) or buccal swabs. Detailed information about sampling can be found here.