Most rat markings are created by mutations at the H-locus. The most distinctive is probably the hooded, but the full variety is quite amazing. Opinions on which alleles actually exist in the fancy at the moment differ, but some known alleles include:
- H – Wild type, minimum white spotting.
- He – Quite extensive white spotting
- hi – Irish, small amount of spotting confined to the underside
- h – Hooded, extensive white spotting confined to the rear end of the animal
- Hre – Restricted, extensive white spotting, lethal when homozygous
- Hn – Notched, quite extensive white spotting
- Hro – Robert type, lethal when homozygous, gives a small amount of white spotting with a fading effect
All of these mutations are co-dominant, meaning heterozygous rats are different varieties from homozygous rats.
There are several other loci which also modify the markings. First of all there’s the downunder locus. This is not scientifically described, but it’s sometimes given the symbol “Du”. The problem is that is already more commonly used for the dumbo locus, so I have chosen to use the following notation:
- hdu – recessive wild type allele, so “no effect”.
- Hdu – dominant downunder allele. The rat has pigment on the underside in a stripe down the centre. This may be broken up or patched depending on the effect of other h-locus alleles present.
Another important one is named the hooded modifier locus, and this controls the length of the stripe in hooded rats – as well as, presumably, having an effect on the amount of pigmentation visible in other non-hooded marked rats:
- Hml – long dorsal hooded pattern. Necessary for hooded rats to have a stripe reaching right down to the tail.
- Hms – short dorsal hooded pattern. Necessary for barebacks (of some genetic types) to have a short or non-existent saddle pattern.
The other really important thing to remember with markings is that we categorise them on what we see. A rat with two copies of the Irish allele might look like an Irish – or it might look like a Berkshire. A rat with one copy of the Irish allele might look like an Irish, but equally it could be a self with a couple of white toes. A rat with the Robert type gene is an Essex, but if there’s no selection for shading then it could look like a perfectly standard Berkshire. For these reasons, there will always be confusion and odd breeding results when dealing with marked varieties, and there will always be arguments and confusion over what particular marked variety a rat actually is.