Modifications to Dark Colors

Color Genetics: Part II of a series

By Tracy Williams

Few species weave such a complicated web of color like the horse. While in our last discussion, we determined the genetic explanation for the three basic dark horse colors, few horses we know in real life can be so simply classified. There are literally hundreds of possible color combinations, but remember that virtually any color can be traced back to the original three: black, bay and chestnut – a truth that can seem confusing as we seemingly stray far from these shades. Our next step in our color journey is to discuss two modifications to our dark colors.


Subtle Adjustment to Base Color

There are three important adjustments to base color: subtle adjustment, dilution and patterns of white. This article will address the first two with a discussion of white patterns in a later story. Subtle adjustment can be broken down into three subcategories: shade, sooty/smutty and mealy. These three adjustments can affect a coat color minimally or so extensively that they obscure the original shade. In addition, they can interact with one another.

Shade
Shade modification, either dark, medium or light, is most noteworthy on bays and chestnuts but black horses can also be mildly affected. Bays can range from a dark, saturated red to a pale, slightly yellow red, and chestnuts can range from a dark liver chestnut to a light, sandy shade. Dark-shaded black horses are called jet or raven, while the lightest hue is a washed out black, often called “summer black”. (See Table 1) Genetic control for shade is controlled by numerous genes and is also subject to the environment. For example, horses can appear lighter in the sun-drenched summers than in the dead of winter.

Dilution to Base Color

The second major modification to dark colors is dilution, which can radically alter our basic shades. In this section we will consider the four dilution genes: Linebacked Dun, Cream-related, Champagne and Silver Dapple.

Linebacked Dun
The linebacked dun gene (Ln) takes our imaginations back to a time of wild, maybe even prehistoric, horses. This gene’s dominant state produces horses with primitive marks: dorsal stripes, leg bars and stripes on withers, much like the horses we imagine wandered the earth before domestication. It lightens both red and black pigments in body hair but leaves the points untouched, turning bay, black and chestnut colors to tan, slate blue grey and light red, respectively. The most common group of duns is called the Zebra dun, diluted bay characterized by black points, primitive marks and tan bodies. The second group, formally called grullo, is diluted black with body colors ranging from beige to slate with black points, dark heads and primitive marks. The final group, red duns, are diluted chestnuts, typically light red with darker red points and primitive marks. (See Table 2) The dun gene can interact with shade and sooty mechanisms to produce more variety.

Two dominant cremello alleles (CrCr) dilute all shades to ivory. Bay and brown become perlino; chestnut becomes cream cremello, and black becomes smoky cream. The classic cream-colored cremello has pink skin, blue eyes and a white mane and tail. A perlino has these attributes but also retains some color in the mane, tail and on lower legs. A smoky cream retains even more pigment in the mane, tail and lower leg and also some on the body. The differences between these three types are often subtle, and it is nearly impossible to detect the base color. Usually there is little shade change and no sooty expression in this category but there can be interaction with the dun gene. (See Table 3).

Champagne
This third dilution gene (Ch) produces a group of beautiful and rare pale colors that can be easily confused with creams. However, they are distinguished by “pumpkin skin” (pinkish, light brown), amber eyes and coat colors that range from chocolate brown to yellow but all with a brilliant sheen and often reversed dappling.

The Champagne gene is completely dominant, so homozygous recessive (chch) produces non-champagne colors while one or two dominant alleles (Chch or ChCh) dilute black pigment to brown and red to yellow. Black becomes champagne, characterized by a pink/beige body with medium to light brown points. Bay becomes amber champagne, illustrated by a light tan body with brown points. Chestnut becomes gold champagne, a similar color to palomino – brilliant golden body with white mane and tail. Palomino becomes ivory champagne, characterized by shiny cream bodies, blue-green eyes and light skin. Any ordinary color can have a champagne variant.

Silver Dapple
The term silver dapple entices an image of a striking dapple grey sailing over a sky-high jump, but this is not a shade produced by this particular gene. While the silver dapple gene (Z) results in some beautiful color combinations, these hues often lack any silver character…and sometimes even lack dapples. The dominant allele only dilutes black pigment, leaving red unchanged. Black becomes either chocolate silver, blue silver or silver dapple, characterized by a flat, brown body color with flaxen mane, tail and lower legs and often heavy dappling. Eyelashes, coarse muzzle and facial hair can be nearly white as well. Bays become red silvers, characterized by a clear bay body color with flaxen points. A hallmark trait of red silvers is that the black point color on the upper legs fades to flaxen towards the hoof. Brown and buckskin become brown silver and yellow silver respectively with similar characteristics. While this is a highly variable group, each subtype consistently has pale eyelashes. Because red pigment is unaffected, chestnut or chestnut-based horses are physically unaffected and can carry the gene unnoticed and pass it to offspring.

When the world of color possibility opens, it can be confusing to determine your horse’s true tint – whether for registration papers, breeding purposes or merely to satisfy your own curiosity. Keep in mind that appearances can be deceiving. To make the most educated guess possible, factor in body color combined with point color, eye color and any known progeny shades, but keep in mind that even with the most calculated conjecture…there is always room for a surprise.

References:

Sponenberg, D. Phillip. Equine Color Genetics. Blackwell Publishing. 2003.

Corum, Stephanie J. “A Horse of a Different Color”. The Horse. May 2003.