The Extension Locus (E Locus for short) influences how much color pigment is on the shaft. Unlike the B Locus and D Locus, the Extension Locus controls how much pheomelanin (red pigment) is produced on the hair shaft. There are four variations on the E allele, but the intensity of each of these variations’ hues is influenced by the elusive rufus modifier and wideband gene.
The Steel ‘E(s)’ Gene
While similar to the chinchilla gene in that it causes ticking, the steel gene does not cause the same color bands that chinchilla does. Instead of a light underband, a dark band, and white, the steel gene removes the undercoat band and causes gold or white bands on the tip. An unpredictable gene, the steel is only fully dominant over full extension ‘E.’ When ‘E(s)’ is homozygous, it is referred to as being super steel, and while super steels may occasionally look like a normal steel, most often they appear to be a dulled self. Similarly, when ‘E(s)’ is paired with non-extension, the coat may have the phenotype of a dull self. If paired with the harlequin gene, steel may cause either a normal looking ticked coat or a coat with patches of ticking and patches of dull self like a harlequin.
GTS Versus STS
When steel is paired with the full color ‘C’ gene, the tips are gold paired with whatever the actual coat color is. This is referred to as being a Gold Tipped Steel or GTS. A rabbit may be a black, chocolate, blue, or lilac GTS.
If steel is paired with the chinchilla dark or chinchilla light gene (‘c(chd)’ or ‘c(chl)’) the tips are silver and the coat color is called Silver Tipped Steel or STS. If the rabbit is known to be either heterozygous or homozygous with c(chl), it is referred to as sable steel or seal steel respectably. Just like GTS, all three forms can come in black, chocolate, blue, or lilac.
Blue Super Steel
The Full Extension ‘E’ Gene
The E, or full extension, gene causes the production of normal levels of pheomelanin and is present in the main agouti variations and self variations. It is recessive to steel, incompletely dominant over the harlequin gene, and fully dominant over the non-extension gene.
The Harlequin ‘e(j)’ Gene
The harlequin gene causes overproduction of pheomelanin on only parts of the rabbits coat and suppresses the production of pheomelanin in other parts. This causes a patchwork of orange and black, chocolate, blue, or lilac all across the rabbits body. The harlequin gene is dominant only over the non-extension ‘e’ gene.
There are many harlequin color combinations, but they can be simplified into three basic patterns: orange-based harlequin, magpie, and tri-color.
The orange based harlequin is simply referred to as harlequin and is always an orange variation (red, orange, fawn, or cream) and one of the basic four colors controlled by the B and D Loci.
Magpies are caused when the harlequin gene is paired with the chinchilla dark or light gene which suppress the production of pheomelanin and leaves white where the orange was supposed to be. Similar to the STS-based colors, there is a magpie, sable magpie, and seal magpie.
Tri-Colors on the other hand, combine the best of magpie and harlequin. When ‘e(j)’ is paired with the broken gene (‘E(n)’), the result is a white based fur spotted with a dark color and an orange.
The Non-extension Gene
The non-extension gene suppresses the production of every color pigment except pheomelanin causing an all orange rabbit. There are a couple variations of oranges, including cream (which is orange diluted) and chocolate orange. The intensity of the orange hue is controlled by rufus modifiers and wideband gene, and the actual vibrancy of the orange can range from fawn to a deep red.
When paired with the agouti gene, a non-extension coat has the agouti white lacing around the ears, stomach, etc..
When non-extension is paired with c(chl) or c(chd) a fascinating and gorgeous coat color is created. Chinchilla dark paired with non-extension causes frosty or pearl; a mostly white rabbit dusted lightly with color on the tips of the fur. When ‘e’ is paired with heterozygous chinchilla light, sable point is the result, and when ‘e’ is expressed with homozygous chinchilla light, seal point is the result.
Another unique combination that arises, is when self (‘aa’) is paired with non-extension. This causes a dark underbelly, face, and ears, and an orange or fawn back and head. This is called a Tortoise (or tort) coat. When a rabbit carries ‘aa,’ ‘c(chd)’ or ‘c(chl),’ and ‘ee, ’ it produces sallander (‘c(chd)’), sable point (‘c(chl)’), or sable point (‘c(chl)c(chl)’).
For phot references, check out this wonderful color guide by Green Barn Farm.