Yes, the 'e' locus is the main component other than the chinchilla gene. The recessive non-extension (fawn) gene
ee is what makes the middle agouti band (orange color) go to the end of the hairshaft. Chin then removes all orange from the hairshaft, leaving just the pearly white. Wideband doubles the width of the middle band, pushing off the dark tips. The two work hand in hand.
According to Oregon State University at
Understanding the genetics behind rabbit coat colors: Part 2 — coat color genes "The
E gene, scientifically known as the
MC1R (melanocortin 1 receptor) gene, controls the extension of dark pigment. . ." All the options on the E gene determine how the dark color extends along the hairshaft, which is why it is called E for Extension. Dominant black (which is quite rare) extends to the end like a self black, steel extends almost to the end with the remnants of the middle fawn band clear out on the tips. Normal extension would be our normal colors like chestnut agouti. Harlequin is next, with the dark color stuck in patches on the skin instead of bands on the hairshaft. The last one is the most recessive, non-extension, where the dark just plain skips the main body hair.
According to
Locus W - Wide Band | rabbit-gentics
"The wideband gene is not discussed often and when it is, it is often confused with the nonextension gene (e), probably because it has a similar effect and the two are usually ‘used’ together. The wideband gene has two alleles. W is dominant, so Ww and WW both produce the normal colors. The ww genotype doubles the width of the ‘red’ bands in agouti rabbits.
The wideband allele is most common in non-extension rabbits (reds - A-eeww and torts aaeeww), where it helps to reduce smut and intensify the red color. It is also common in coppers - along with high rufus factor it helps to distinguish the copper from the chestnut agouti.
In chinchilla rabbits (A-chd-ww), the wideband gene doubles the width of the silver-white bands, resulting in rabbits that are lighter and less even than normal. In other agouti rabbits (A---ww) the bands are also wider and the red shade may be more intense as well. In general, wideband is accepted but not preferred in the chinchilla and other agoutis. Since the selfs and tans don’t have bands, the wideband effect is not seen, and is just carried."
According to Minifluffs Caviary at
Rabbit Color Genetics Chart
"Technically, the wideband gene is a recessive modifier - ww is used in combination with agouti (A-) and non-extension (ee). So The wideband modifier genotype ww does two things, it lightens the undercolor and doubles the width of the midband. The latter may seem to be redundant as the ee genotype should remove all eumelanin, but wideband group colors with a stray Ww or WW are invariably 'smutty' with eumelanin tipping on the haunches and ears and usually have a grayish tint to the undercolor."
So, if this information is correct, wide band would explain both the excessively light fiber on my chins, and the pale (white) undercolors I am dealing with. All my chestnut agoutis are the "wild gray" pattern, a mostly gray pattern with little of the fawn band, and they have white undercolor instead of the standard blue. I'd heard once that wild gray was a wide band pattern, although there must be another modifier because all the wide band chestnuts I've seen had an extra wide fawn band, not extra gray. Such fascinating mysteries.
Since I'm breeding mostly for fiber quality, progress on the colors has been slow.