A black mask should occur in all cases. The fawn is generally a light "silver" shade, but may range up to a golden yellow. The apricot may be a slightly reddish hue up to a deep, rich red. The brindle markings should ideally be heavy, even and clear stripes, but may actually be light, uneven, patchy, faint or muddled. Piebald Mastiffs occur rarely. Other non-standard colours include black, blue brindle, and chocolate (brown) mask. Some Mastiffs have a heavy shading caused by dark hairs throughout the coat or primarily on the back and shoulders. This is not generally considered a fault. Brindle is dominant over solid colour, other than black, which may no longer exist as a Mastiff colour. Apricot is dominant over fawn, though that dominance may be incomplete. Most of the colour faults are recessive, though black is so rare in the Mastiff that it has never been determined whether the allele is recessive or a mutation that is dominant. The genetic basis for the variability of coat in dogs has been much studied, but all the issues have not yet been resolved. On the basis of what is known (and remembering that, as dogs are diploid animals, each gene location (locus) appears twice in every animal, so questions of dominance also must be resolved), the gene possibilities allowed by the Mastiff standard are AyBDEmh(kbr_or_ky)mS. This describes a dog which is fawn with a dark nose, non-dilute, black-masked, non-harlequin, brindled or not brindled, non-merle, and non-spotted. To allow for the rare exceptions we must include "b" (brown mask and possible brown brindling), "d" (blue mask and possible blue brindling), "sp" (pied spotting), and perhaps "a" (recessive black). The possible combination of homozygous brown and homozygous blue is a pale brown referred to as Isabella in breeds where it is relatively common. On a Mastiff, this would appear on mask, ears, and any brindling that was present. Speculative gene locations may also exist, so a Mastiff may be "I" (apricot) or "i" (non-apricot) and perhaps "cch" (silver lightening) or "C" (without silver lightening). (Note that this "C locus" may not be the same as the one identified in other animals, SLC45A2.) Record size The greatest weight ever recorded for a dog, , was that of an English Mastiff from England named Aicama Zorba of La Susa, although claims of larger dogs, including Saint Bernards, Tibetan Mastiffs, and Caucasian ovcharkas