How does the D-series affect masks in Fawn Danes?

The D-series (Dilute or MLPH gene) affects the masks on Fawn Great Danes by lightening the dark pigment (eumelanin) that creates the mask.

While the presence of the mask itself is controlled by the E-series (specifically the E 

M

  allele), the D-series acts as a global modifier for how intense that black pigment will be. If a Fawn Dane inherits two copies of the recessive dilute allele (d/d), the D-series will dilute all of the black pigment on the dog’s body.

As a result, the dog’s genetic “black mask” will be visually diluted into a blue (grey) mask. This same genetic interaction is also what causes the black stripes on a Brindle Dane to appear blue. A Fawn Dane that exhibits this diluted blue mask is commonly referred to by breeders as a “blue fawn”.