Dominant black in horses(1).pdf

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Dominant black in horses

DP Sponenberg

MC Weise

Department of Biosciences and Pathobiology,

Virginia-Maryland Regional College of Veterinary Medicine,

Virginia Tech, Blacksburg, VA 240611-

2714 Wocjik Ln, Sobieski, WI 54171, USA

(Received 20 January 1997; accepted 7 March 1997)

Summary - The existence of dominant black in horses is supported by a black stallion

producing 12 black or near black and no other color of foals from bay mares, and 16 black

or nearly black and no other color of foals from chestnut mares. This allele is suggested

as being dominant black, , at the Extension locus. This allele does not always cause

completely eumelanic phenotypes, since some offspring (which were heterozygous) were

near black rather than completely black. The dam of this stallion was of a near black

or brown, rather than black, phenotype. The sire of this stallion was black. Foals were

sometimes born a color close to that of bay foals, but these ultimately turned completely

or nearly black at maturity.

color inheritance / Extension locus / dominant black / horse

Résumé - Noir dominant chez les chevaux. L’existence d’une coloration noire dominante

chez les chevaux est mise en évidence par un étalon noir produisant 12 poulains de couleur

noire ou presque à partir de juments bai et 16 poulains de couleur noire ou presque noire

à partir de juments alezan. Cet allèle est suggéré être le noir dominant, , au locus

Extension. Cet allèle ne cause pas toujours des phénotypes eumélaniques puisque certains

descendants (qui étaient hétérozygotes) étaient presque noirs plutôt que complètement

noirs. La mère de cet étalon était presque noire ou brune, plutôt que noire. Le père de

cet étalon était noir. Les poulains naissaient quelquefois avec une couleur proche de celle

des poulains bai mais devenaient complètement ou presque noirs à la maturité.

hérédité de la couleur / locus Extension / noir dominant / chevaux

INTRODUCTION

The inheritance of color in horses has been studied for much of this century. Early

studies were largely based on studbook data, and the accuracy of subtle distinctions

in color nomenclature were sometimes lacking (Gremmel 1939, Castle 1940a, b,

Correspondence and reprints: Veterinary College, Virginia Tech, Blackburg, VA 24061

USA

1951a, b, 1954). This is especially so when horses are registered as foals, since foal

coats and adult coats can be very distinct from one another. Later studies were

mostly based on segregation data from selected families of horses, and provided

a sound basis for the testing of hypotheses concerning the inheritance of horse

color. These studies also tended to use the principles of homology in assigning loci

and alleles for the various genetic phenomena controlling color (Odriozola, 1951;

Adalsteinsson, 1974, 1976).

Black horses are usually considered to be uniformly covered with black hairs, and

in many breeds the presence of even a few reddish or tan hairs (usually in flanks

or muzzle) results in the designation of such horses as brown, or even bay, rather

than black (Sponenberg, 1996). Horses with reddish brown bodies and black lower

legs, manes and tails, are consistently designated as bay in most breed registries.

While such red horses are consistently referred to as bay, the term bay can also

include darker colors, some of which approach black. Horses with a mixture of

black and reddish brown body hairs and black lower legs, manes and tails are

variably considered brown or bay, depending on the breed or the observer. The

brown classification can therefore include horses that are nearly black, as well as

those that are nearly bay. An additional type of brown is seal brown, which has black

lower legs, mane and tail, and a black body with conspicuous lighter hairs on the

muzzle, over or around the eye sockets, and in the flanks and axillary regions. The

genetic differences between these various sorts of brown have never been elucidated,

largely because the nomenclature in the English language is ambiguous enough

that it is difficult to accurately designate horses into one color group or the other

(Sponenberg, 1996).

Few studies have been accomplished that finely split the various browns accu-

rately. Abeles (1979) presents an hypothesis for the interrelationships of horses with

black lower legs, manes and tails, and bodies of reddish brown, mixed reddish brown

and black, or uniform black. Although no segregation data are presented, her expe-

rience is consistent with the reddish brown and black mixtures (which she refers to

as brown) dominating the clear reddish browns (her bay), which in turn dominate

the uniformly black bodies (her black). This is perhaps explained by the interaction

of the Agouti and Extension loci. These loci have long been postulated as impor-

tant in controlling common horse colors, although the hypotheses concerning their

interactions have varied over the years (Odriozola, 1951; Jones, 1982; Sponenberg,

1996).

The existence of a uniform black in horses that is recessive to bay has long been

acknowledged, and accounts for the majority of black horses. This black is assigned

to the Agouti locus in keeping with principles of homology (Wilson, 1910; Wright,

1917; Adalsteinsson, 1976; Lauvergne et al, 1991).

The existence of a dominant black in horses has also long been speculated

(Gremmel, 1939; Castle, 1940a, b, 1951a, b, 1954). The evidence for a dominant

mechanism producing black has been the production of bay or brown foals from

the mating of two black parents. Such instances have usually been from studbook

data, which are suspect owing to the possibility of inaccuracies in the registrations

of animals, or in mistakes of parentage. Another problem with studbook data

for the colors bay, brown and black is one of definition. Various breeders may

have different mental images of these three and consequently studbook records are

probably inaccurate at this level of detail. Dreux (1966) presents evidence for the

existence of a dominant gene contributing to black or dark bay phenotypes in the

French Trotter breed, and mentions that this is rare in horses of European breeding,

having been introduced through an imported American Trotter stallion.

Currently, documentation best fits with the view that the Agouti locus comprises

two alleles, with bay dominant to black (Sponenberg, 1996). A third allele, black and

tan (!4! is fairly poorly documented as a cause of seal brown, and is intermediate

in dominance to the other two alleles. The Extension locus likewise has two

well-documented alleles, wild type dominant to chestnut (Sponenberg, 1996). The

recessive chestnut allele, when homozygous, results in a uniformly reddish brown

phenotype (including lower legs, mane and tail), and is epistatic to the Agouti

locus. Past literature has postulated intricate relationships with these two loci,

with no clear epistasis of one over the other, and with these intricate interactions

explaining the differences between bay and brown, and between shades of chestnut

(Jones, 1982). The Extension locus may well be the site of an allele that provides

for the intermixture of black hairs into the red body coat of some bay horses, which

translates into the difference between bay and brown. This is consistent with Abeles

(1979), even though segregation data are lacking in that report.

This report discusses data that support the existence of an additional Extension

allele, for dominant black, in the horse.

MATERIALS AND METHODS

Production data from a black Arabian stallion (Serr Ebony Star) were evaluated

for foal color production with various colors of dams. The production data from

the black sire (Shilosmidnight) and the dark brown dam (PRF Gali Gaeraff) of this

stallion were similarly evaluated.

RESULTS

Results are summarized in table I. The black stallion was mated to mares of

various colors. Following mating to bay mares he produced seven black and five

’near black’ foals (’near black’ is used to describe horses that are black save for a

few tan or red hairs in muzzle or flank, or those horses that are very dark but for

which it is possible to discern a subtle difference in the color of body and the lower

legs, mane and tail). Following mating to black mares he produced nine black foals.

Following mating to chestnut mares he produced ten black and six ’near black’

foals.

The sire of this stallion was black and was mated to black mares to produce

two black foals. Matings to bay mares resulted in three black and three bay foals.

Matings to chestnut mares resulted in one black and two bay foals.

The dam of this stallion was dark brown, and was mated to a black stallion to

produce one black foal and three ’near black’ foals. A mating to a chestnut stallion

produced a ’near black’ foal.

DISCUSSION

The problem of nomenclature concerning dark horses is a concern for this study.

In the Arabian breed many breeders are very strict in their use of the term black,

and reserve it for horses with black hairs only, excepting white marks due to spotting

patterns. Arabian breeders do not use the term brown, and so horses that are nearly

but not quite black are usually referred to as black bays, or even simply as bay.

These are, however, very distinct from the usual bright reddish brown bay that

is typical in the breed. Black bays in the Arabian breed are easy to confuse with

black horses, especially without close and detailed examination, and are difficult to

confuse with bright reddish brown bay horses, which are common in this breed. The

grouping of blacks and near blacks, or black bays, as one group is therefore logical,

since this is how these colors appear to most observers. These three are very close

to one another visually, if not in terms of nomenclature.

The results of the mating of the horses of this study is consistent with the

segregation of a dominant allele for black. The mating of the stallion to bay mares

for 12 foals can be tested for goodness of fit for the hypothesis that this stallion is a

recessive black. If that were the case, and if all the bay mares were heterozygous for

this usually rare allele, then 2

12,1 df, P < 0.005. This deviates significantly

from the expectation, even postulating the most favorable condition of all the bay

mares being heterozygous for a rare allele. These results are therefore inconsistent

with the segregation of a recessive allele for black, but are consistent with the

stallion being homozygous for a dominant allele causing black color.

The sire of this stallion was black, while the dam was dark brown. Both needed

to have had the allele causing dominant black if the stallion is indeed homozygous,

as is supported by the segregation data. The mare always produced black or near

black foals when mated to a black stallion. She also produced a near black foal

when mated to a chestnut stallion, even though she is an obligate heterozygote for

dominant black by virtue of having had a bay sire. The sire of the stallion produced

equal numbers of bay and black foals when mated to bay mares, which is further

evidence that a dominant gene is segregating in this family of horses. The sire of the

stallion is heterozygous for this gene, even though both of his parents were black.

A few of the foals in this study were born a color resembling that of foals that

ultimately prove to be bay. These foals darkened with age, ultimately becoming

black or nearly so. Some of the blackest of the horses in this study were born with

this light bay foal coat, only to darken later. It is fairly typical for black horses

(assumed to be of the recessive Agouti genotype) to be born an ashy grey color, to

darken later. Whether or not the differences in foal coat are consistent with these

two different genetic mechanisms that cause black horses is undetermined.

The allele causing the black phenotype in these horses is most likely the dominant

black () at the Extension locus. This allele is documented in numerous other

species as a cause of uniformly eumelanic phenotypes (Searle, 1968). It appears

to be arguably incomplete in its action in some instances, causing dark brown or

near black instead of truly black individuals. In foxes, for example, the D allele

is frequently incompletely dominant resulting in three color classes from this allele

and the wild type allele (Adalsteinsson et al, 1987). The dominant black allele may

be the genetic mechanism behind some of the very dark phenotypes in horses that

are not truly uniformly black. The differences in genotype between the black and

nearly black horses are uncertain, since in this data set it is conservative to assume

that most, if not all, of these had the Agouti locus bay allele, and its presence or

absence can therefore not be responsible for the subtle differences in the black and

near black horses. Likewise, most of these foals had to have been heterozygous for

the dominant black allele, and this genotype includes both black and near black

individuals. The dominant black allele appears to be rare in most breeds, since in

most breeds black does segregate as a recessive.

REFERENCES

Abeles HM-S (1979) A coat of many colors. Equus 17, 30-38

Adalsteinsson S (1974) Inheritance of the palomino color in Icelandic horses. J. Hered. 65,

15-20

Adalsteinsson S (1976) Colour inheritance in Icelandic Ponies. In: First Int. Synzp. on

Genet. and Horse Breed., Royal Dublin Soc. Dublin, 17-18 September 1975, 42-49

Adalsteinsson S, Hersteinsson P, Gunnarsson E (1987) Fox colors in relation to colors in

mice and sheep. J. Hered. 78, 235-237

Castle W (1940a) Mammalian Genetics. Harvard University Press, Cambridge, Mas-

sachusetts

Castle W (1940b) The genetics of coat color in horses. J. Hered. 31, 127-28

Castle W (1951a) Dominant and recessive black in mammals. J. Hered. 42, 48-50

Castle W (1951b) Genetics of the color variety of horses. J. Hered. 42, 297

Castle W (1954) Coat colour inheritance in horses and in other animals. Genetics 39,

35-44

Dreux P (1966) Contribution a 1’etude du gene E chez le cheval domestique. Ann Genet,

9, 168-170

Gremmel F (1939) Coat colors in horses. J. Hered. 30, 437-45

Jones WE (1982) Genetics and Horse Breeding. Lee and Febiger Publishers, Philadelphia

Lauvergne JJ, Silvestrelli M, Langlois B, Renieri D, Poirel D, Antaldi GGV (1991) A new

scheme for describing horse coat colour. Livest. Prod. Sci. 27, 219-229

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