Silver, which is a scattering of white guard hairs on the back, especially around the shoulders. All or nearly all Norway rats have some silvering: there is no known single gene which controls the degree of silvering, but some strains of rats are much more heavily silvered than others. For historical reasons black rats with silvering are called silver greys, and all fawns are called silver fawns whether they have visible silvering or not. Silvering shows up best on dark colours, and is most marked in young adult bucks.

Husky, producing a scattering of white hairs so intense that they make up a significant proportion of the coat and change the apparent colour, so that e.g. black huskies look charcoal-grey. Husky rats are born with plain, un-roaned coats, and the silvering appears almost overnight at around seven weeks. They always or nearly always have a white belly and muzzle and a blaze - often a broad, triangular badger-face blaze with the point extending almost to the crown of the skull. The eyes have a ruby retina and a blue-tinged iris.

As they age they turn white from the nose back, until the entire face is white and the blaze is lost. White also spreads from the belly up the sides. In young animals the white belly may have uneven, wandering edges, but as it spreads it straightens out and takes in the limbs and throat as well. A middle-aged husky rat looks as if someone took a plain white animal and then painted a perfectly straight two-inch wide coloured stripe from the crown of its head to its tail.

The hood series, in which the distribution of the melanocytes which produce skin pigmentation is restricted to varying degrees in the embryo. These melanocytes spread from the central nervous system to the rest of the body: the marked patterns progressively withdraw colour from the body back towards the spine and skull.

The genetics of the hood series are complex and involve combinations of several co-dominant genes: e.g. breeding hooded to self produces Berkshires. Nor are these markings purely controlled by genes. Masked, black-eyed white and odd-eyed-white rats are all homozygous for the Extreme Hooded gene, which restricts colour to the front of the skull: but it seems to be mainly chance and developmental factors which decide whether the animal will have colour only in the eyes or right across the face.

The stages of the hood series are as follows:

In variegated, hooded and bareback animals the colour on the shoulders may or may not also extend down the forelegs to the wrist, though for show purposes the forelegs should be white. Many of these animals have odd spots of colour on the sides or belly which are considered a fault for showing. In particular, bucks often have round coloured dots over the ureters, to either side of and about half an inch in front of the penis (this area of skin is very thin and sometimes ulcerates in very old males).

Hooded animals often have a very irregular stripe, and/or blotches on their sides. In extreme cases you can get an animal with large coloured patches, resembling the pattern which in other animals is variously called broken, piebald/skewbald or paint. However there are no known true broken-marked fancy rats at present. A combination of the hood and chinchilla genes does produce true broken-marked or patched rats, but the combination is semi-lethal: most of these animals have malformed guts and die very young.

Chinchilla is currently regarded as part of the Hood or White Spotting series, but I personally find it very hard to believe that chinchillation really is part of the hooded sequence, since it does something entirely different. All other genes on the H locus modify the distribution-pattern of melanocytes in the skin, whereas in chinchillas the melanocytes are evenly distributed but lose their ability to produce red pigment (phaeomelanin).

In some black capped and black masked rats - hh^e and h^eh^e - the Extreme Hooded gene h^e produces a slight chinchillation effect, so that the markings come out a greyish chocolate colour. But some capped and masked rats have fully black markings. This indicates that the fading effect is not due to the h^e gene per se but to an unidentified linked gene. I suspect chinchillation is a Hood-series gene producing a white belly, initimately linked with another allele of the gene which causes faded colour in capped rats.