As well as the main genetics we know contribute directly to coat color, there are several polygenes (meaning more than one gene is involved) which affect coat color intensity, undercolor, and more. These polygenes are often referred to as 'modifiers', because they modify the actions of other genes. Additionally, there are things we understand to be genetic but don't understand how they are inherited yet. This page will cover both types of factors on coat colors.

Rufus Factor (AKA Rufus Modifiers)

Rufus factor is the name for a set of polygenes which control the intensity of phaeomelanin (red-yellow) pigment. They are additive, in that the more rufus modifiers are affecting the coat color, the more intense the phaeomelanin expression is. Low rufus factor results in light phaeomelanin, such as fawn or even a very pale almost white cream. High rufus factor results in deep, intense red-orange color. Unless a line or breed has been selected for either very low or very high rufus factor, most rabbits come in somewhere in the middle. 

It is easy to see the actions of rufus factor on colors like chestnut, orange, tortoise, and otter, where the other genetics in play have separated out the phaeomelanin and eumelanin (brown-black pigment) in visual ways. In chestnuts, it causes the two pigment types to form distinct bands, and one can easily see the intensity of the phaeomelanin in the intermediate band of the coat. In orange and tortoise, with most of the eumelanin removed from the coat and only the red-yellow pigment left, there can be truly striking differences! In otters, high rufus factor is evident in the tan pattern markings, whereas a low rufus factor otter might look almost like a silver marten.

Rufus factor appears to be linked with the Wideband gene. Rabbits with the Wideband mutation (w) tend to have more intense rufus factor, while rabbits with the non-Wideband allele (W) tend to have lower rufus factor, though not always! 

It is important to note that rufus factor does affect coat even when you can't directly see it! Self rabbits (like black, blue, chocolate, and lilac) still have phaeomelanin in their coats, and the intensity of it contributes to the overall intensity of the color. Truly intense blacks, for example, tend to have strong rufus factor, while poor color blacks often have low rufus factor (or else they may be self chinchilla, because the cchd mutation prevents the expression of phaeomelanin in the coat entirely!).

Also important  to note is that dilute (d) also dilutes phaeomelanin, so dilute colors will fade to a more fawn color. When comparing rufus factor, it is best to compare dilute to dilute and non-dilute to non-dilute for this reason!

Smut

Smut is the name for eumelanin (black-brown pigment) left in the coat of a non-extension rabbit like an orange, red, or toroiseshell. The eumelanin gets 'pushed' to the tips of the fur and creates black, blue, chocolate, or lilac ticking at the  very ends of the hair shaft. This is genetic, though we don't know the exact cause of it.

Smut is also the  cause of the variation in how much color is found on frosty (also called ermine or frosted pearl), sallanders, seal points, and sable points. A heavily 'frosted' frosty just has a lot of smut!

Brindling

Brindling refers to the tendency of the different colored hairs on a genetically harlequin (ej) rabbit to intermix evenly rather than making clear, distinct markings. It does seem to have some genetic component, as brindled rabbits are more likely to produce brindle offspring, and 'clean' (low to no brindling) rabbits are more likely to produce clean offspring. However, the exact mechanism is not understood, and two brindled rabbits can produce clean ones or vice versa.

Other Color Intensity Factors

Unknown color intensity factors also affect eumelanin (black-brown pigment). We know this because self chinchilla colors (which have no phaeomelanin for rufus factor to work with) can vary in color intensity. Likely they are genetic and additive, similar to rufus factor, but we do not know exactly how they were.

Amount and Placement of White

Unknown genetic factors affect the amount of white accompanying white marking mutations (broken, Vienna marked, White Ear, Dutch), as well as the exact placement of the white. These factors are clearly genetic, in that rabbits are more likely to produce similar amounts of white as they have. However, they are not well-understood and may vary from one type of white marking allele to another. In other words, the factors affecting the amount and placement of white in a broken may not be the same ones affecting the amount of white in Vienna marked, White Ear, or Dutch. Keeping in mind, of course, that many of the white marking mutations also produce more white when there are two copies of it (homozygous). 

Band Width

While Wideband (w) is a known factor affecting the width of bands in agouti rabbits, non-Wideband colors  and Wideband colors can both have varying band widths. This is genetic, in that rabbits tend to produce offspring with similar width bands as themselves, but the mechanism is not yet known. Some or all of the factors affecting band width may be the same as those affecting the amount of smut on non-Extension color rabbits. 

Snowballing

Snowballing refers to the white factor which appears in young animals and disappears with maturity. It is most often found in dilute colors. The most obvious snowballing effect is white undercolor in colors which normally do not have white undercolor. The white may extend far up the hair shaft and may have stray white hairs, both of which disappear with age. Less severe cases of snowballing may only have a few stray white hairs or a slightly lighter undercolor than normal, again both of which disappear with age. This should not be confused with true white undercolor, which does not fade away, and is the ideal for lynx color but no other agouti colors. In non-dilute colors, the rare appearance of snowballing is more likely to manifest in a 'mealy' muzzle (a few stray white hairs on the muzzle, near the whisker bed). As before, these go away with age. Snowballing should not be confused with the temporary white tipping found in sable-based colors (including smoke pearls),  the white-tipped hairs often found in young silver martens, or the white tipped hairs found on silver-tipped steels.

Stray White Hairs

Also known as Scattered White Hairs, this is the appearance of fully white hairs in an otherwise colored coat. They do not disappear with maturity and instead tend to appear as a rabbit matures, which the first stray white hairs typically appearing after the rabbit is already several months old. They may continue to increase in number as the rabbit ages. With stray white hairs, there is no other genetic explanation (e.g., snowballing in  young animals, white hairs from the White Ear (We) mutation, silvering from the Silver (si) mutation). Breeding two rabbits with stray white hairs tends to increase the number of stray white hairs in the offspring. This phenomenon is definitely genetic, so animals with stray white hairs should not be bred if your aim is producing show quality rabbits. Some breeders cover up the stray white hairs by breeding affected rabbits to whites (REW or BEW), Californians, Frosties, and other light colors. Unfortunately, this can come back to bite them (or other breeders who buy their rabbits!), because the light colored rabbits still have the stray white hairs, but they are just less (or even not) visible.

Random White Spotting