Equine Color Genetics

D. Phillip Sponenberg, Rebecca Bellone

• Publisher: Wiley-Blackwell
• Year: 2017
• ISBN:
  9781119130611
• Edition: 4
• Topic: Medicine
• Subtopic: Equine Veterinary Science

Equine Color Genetics, Fourth Edition presents a detailed examination of the color variation in horses and donkeys and the genetic mechanisms that produce color variations.

• Thoroughly covers the basic colors in horses, including bay, chestnut, black, and brown
• Details the genetic basis of the colors built from the basic coat color, including dilutions and white patterning
• Provides an explanation of genetic mechanisms that determine coat color
• Presents a thorough revision and update, including new advances in molecular genetics, biochemistry, molecular mechanisms, genetic loci, coat colors before domestication, and more
• Offers a new introduction describing the principles of genetics and genomics research to help outline how knowledge is discovered and to assist the reader in understanding concepts covered in the book

1
Introduction

This book is intended to be a complete discussion of horse and donkey colors. It includes details of color identification as well as their genetic control. The goal is to include all color variations occurring throughout the world and to fit these into a framework that is based on traditional American nomenclature as well as on the genetic phenomena controlling the color variations.

Identification and definition of horse color are important for several reasons, and each of these reasons demands a different organization and presentation of the material. Reasons for accurate horse color classification include identification of individual horses for legal purposes, health records, and breed registrations. In addition, breeders who are interested in producing or avoiding specific colors of foals find that accurate identification of colors in their breeding stock is essential to their success.

The organizational structure of this book combines a strictly visual approach (what color the horse appears to be) with the genetic control (how that color was produced by the interactions of the genes involved). Each section starts off with the visual approach and then delves into the genetic aspects. Unfortunately, a few specific details of horse color are better understood by first explaining the underlying genetic mechanisms that give rise to the colors rather than from any other point of view. In those few cases genetic mechanisms are presented first with the visual aspects following.

The genetic approach to understanding horse color is becoming increasingly common as deoxyribonucleic acid (DNA)-based tests for many of the genes causing the colors are now available. The results of these tests help breeders to better understand the colors and the genetic basis of their production, as well as the range of colors a specific horse can produce. While genetic testing has been generally helpful, it has also revealed a few confusing issues. Some horse colors, when classed only by visual appearance, appear to be a single group, but this single group includes the results of several very different genetic formulas. This occurs with black horses, and also with some of the light colors such as champagne and pearl, as well as silver dapple and mushroom. These different genetic formulas are presented at length in the corresponding sections of the book and are examples of the complexity of the genetic systems that produce a horse's final color.

New instances of multiple genetic formulas leading to visually identical colors are regularly coming to the attention of researchers in this field, and these can easily cause confusion to owners of horses with some of the variants. These colors present a very real challenge because a single outward appearance can spring from very different genetic mechanisms. In most cases it is fortunate that only one of the several possible genetic mechanisms for a confusing set of colors is common, and the few others that are possible are much more rare. Consequently, discussion of the colors can still proceed from the basics of the most common mechanisms, even though the more rare mechanisms that lead to similar visual results must also be considered for completeness. Those that are known are presented here.

Importantly, even though some mechanisms are common and others rare, when a rare mechanism is present it becomes the only one of importance in understanding how a specific horse or donkey will produce color in a breeding program. In a very real sense, “rare” is only really an issue at the population level. At the level of the individual horse the only thing that is important is the specific blend of individual genetic variants, regardless of their frequency in a population.

Choosing a set of nomenclature for horse colors presents an interesting array of challenges. Historical approaches were of necessity based solely on visual classification. These older, traditional systems were detailed and technical and have served well for centuries. They were based on a rich lore of horse-specific information and were generated by people closely familiar with horses and their variation. This traditional approach deserves great respect for having served well for so long.

From a strict and non-equine viewpoint, nearly all horse colors could simply be described as a shade of brown or black or a mixture of those two. People with no equine background do indeed tend to lump most horses as “brown,” because to the inexperienced eye they indeed are. One step beyond the “most horses are brown” approach is the traditional equine-specific nomenclature based on various details related to visual appearance. While traditional nomenclature varies region to region, it has served well for identifying horses by color. It has also served as the framework under which genetic investigations were first accomplished.

In recent years, an approach based more on genetics has come into vogue for understanding and classifying horse color. This approach can often simplify nomenclature. This is especially true when similar phenotypes are caused by mutations in the same gene. However, the genetic approach can also complicate nomenclature, as novel genes or mutations are given new names that have no equivalent in the more traditional system. This can muddy the identification of horse colors.

The presence, for some colors, of multiple genetic mechanisms causing visually similar results presents very real problems for developing a consistent nomenclature based on genetic information alone. Nomenclature can either be visually based or genetically based, but choosing either one of these as the primary organizer of nomenclature will inevitably cause problems for situations in which the other basis is a more compelling consideration. Still, the historic, visually based approach is the only one that is likely to succeed in field situations where genetic details are only rarely known. In contrast, the genetically based approach can be more useful in classing and defining the color of breeding horses when their owners are preferentially interested in producing specific colors of foals. This guide generally uses a visually based approach, but the more genetically based approach is referred to when appropriate.

Colors are discussed in a sequence that first examines dark colors because these tend to be the most common colors in most breeds and most regions. An understanding of less common (generally lighter) colors is then built as a progression from the dark colors. Each section first defines and classifies a color or group of colors by the visual approach and then delves into the details of what is known about the genetic control.

The addition of white hairs can be superimposed on any background color, and each pattern of these white hairs or patches is examined after all basic colors are considered. The patterns of white hairs are organized in the same fashion of first classifying each pattern by its visual appearance, followed by an explanation of the underlying genetics.

Donkey colors are the subject of a separate discussion following the section on horse colors. The donkey color discussion is organized in a manner similar to that of the horse color section, but is shorter. Much less is known about donkey color than is known about horse color, although this body of knowledge has recently begun to expand. Subtle details of donkey color are understood more readily when considered in the light of horse color identification and genetics, which also makes a shorter discussion appropriate.

Mule colors are omitted, except for a few examples. This is due to mule colors being somewhat less well understood than those of horses and donkeys, while at the same time being generally consistent with the expected interactions of the genes controlling color in the two parent species.

A series of summary tables is presented in Chapter 13, after the text and illustrations. Table 13.1 is a list of color names that are included in the text, serving as an attempt at a reasonably complete single list of horse color names and their main distinguishing features. Table 13.2 is a similar list for the patterns of white hairs. Table 13.3 lists the various genes affecting horse color and their actions. This includes both the loci and alleles. Table 13.4 is similar to Table 13.3, but is devoted to the genes associated with patterns of white. Table 13.5 lists genotypes of the different colors so that breeders can more adequately understand them and predict the possible color outcomes from mating various colors of horses. Table 13.6 outlines the various alleles present in different breeds. It can be used by breeders to develop the potential array of colors in various breeds. Table 13.7 is a large and cumbersome table that outlines the potential results of mating various parental colors. Table 13.8 has the details for horse names, breeds, and sources of photographs and other figures.

1.1 Basic Horse Color Identification

One purpose for understanding horse color is to be able to identify horses accurately. Accurate identification of horse color is a key ingredient in understanding the genetic or biologic basis of color and is the foundation upon which genetic investigations are built. Even a casual observer soon realizes that horses have a wide variety of colors. A standardized classification is necessary to begin communicating subtle differences between some specific horse colors. Any standardized system of color nomenclature depends on observers ...