The Role of Genetics in the Horse Enterprise

Introduction

The genetic composition of the horse is controlled by genes which are located on 64 chromosomes, 31 pairs of autosomal chromosomes plus the sex chromosomes, XX in females and XY in males. Many traits of horses such as mature size, growth rate, speed and body conformation are influenced by large numbers of genes found on many chromosomes and are also influenced by nutrition, disease, training methods, climate and other environmental effects. Other traits, however, such as coat coloration, some genetic defects, and blood types are controlled by single pairs of genes and are generally not influenced by the environment. Before continuing with a discussion of these types of traits, which can be referred to as qualitative traits, it is perhaps useful to review some genetic terminology.

Nearly all the genes to be discussed in this chapter are located on the 31 pairs of autosomal chromosomes and thus, there are two genes at each locus (a point on a chromosome) in normal horses, one on each of the paired chromosomes. A locus can be defined as the point on a chromosome at which a particular gene is located. Different genes which can be found at the same locus are called alleles of each other. Usually we are not aware of the existence of a particular locus until a mutation (change from the original, normal state) occurs. Thus, the existence of a locus has generally been determined in the past through the presence of the mutant gene. In other words, the mutant gene which causes a particular change in the animal shows what the normal gene at the locus was responsible for producing. However, in the future, as molecular biology continues to advance, the sequence of normal genes will be determined directly through study of the DNA sequences on the chromosomes.

The normal gene and mutant genes found at the same locus are said to be alleles of each other. The mutant gene may be dominant, incompletely dominant or recessive to the normal gene at a particular locus. If the mutant gene is recessive, as is the case for most genes which cause genetic defects, an animal can have one normal and one mutant gene (a combination which is called being heterozygous) but will still appear normal. Such a heterozygous animal is also often referred to as being a carrier of the mutant gene. An example of a recessive mutant gene is the gene responsible for the lethal condition, combined immunodeficiency (CID), in Arabian horses. Foals which are heterozygous (carriers) for the gene responsible for the CID condition are perfectly healthy as the gene responsible for the CID condition is recessive. The genotype of such a horse, symbolic indication of its genetic composition can be written Cid⁺/cid, indicating that it has one normal (Cid⁺) and one mutant (cid) gene. Only when a foal receives two cid genes, one from each parent, is it afflicted with CID.

The effect of recessive genes on the phenotype (the actual appearance or performance) is observed only when they are homozygous; that is, when a horse carries two recessive genes of the same type. When two horses which are both heterozygous (carriers) of a recessive gene, cid for example, are mated, there is a 25% chance that a foal which is homozygous for cid, (cid/cid in genotype) will be born. This can be shown in the following simple diagram.

Sperm from a Carrier Stallion (Cid⁺/cid)

Ovum from a Carrier Mare

(Cid⁺/cid)
50%

Cid⁺
50%

cid

50% Cid⁺ 25% Cid⁺/Cid⁺ foals

(Homozygous normal)
25% Cid⁺/cid foals

(Carriers)

50% cid 25% Cid⁺/cid foals

(Carriers)
25% cid/cid foals

Affected, foals die

due to CID

It should also be clear from this diagram that in addition to the 25% affected foals which will die, 75% will be expected to be normal, healthy foals and two-thirds of these (50% of all foals born) will be expected to be carriers like their parents.

When a carrier stallion (heterozygote) is mated to homozygous normal mares (Cid⁺/Cid⁺), all the foals will be phenotypically normal -- all will appear healthy, but half of the foals from such a mating will be carriers of CID. This situation is shown diagrammatically below:

Sperm from a Carrier Stallion (Cid⁺/cid)

Ovum from a Normal

Mare
50%

Cid⁺
50%

cid

100% Cid⁺ 50% Cid⁺/Cid⁺ foals

(Homozygous normal)
50% Cid⁺/cid foals

(Carriers)

Thus, a breeder should be concerned about the genetic makeup of a horse whose sire or dam was known to be a carrier of a recessive genetic defect.

In general, the only way that we become aware of those animals which are carriers is through their production of a defective progeny. An animals which is homozygous for a recessive gene must have received a recessive gene from each of its parents and thus, both must have been carriers. An example of this rule in horse coloration is the sorrel coloration which is recessive to bay. If a sorrel foal is born to bay parents, both parents have to carry the recessive gene responsible for the sorrel coloration.

Examples of mutant genes which are dominant are those controlling the gray coloration and the tobiano paint pattern. If a horse carries the mutant gene responsible for the gray coloration (symbolized by G) and its normal (nongray) allele (symbolized by g⁺) it will be gray. Whenever the gray gene is carried by a horse, whether homozygous (GG) or heterozygous (Gg⁺) it will express the gray coloration. Thus, any gray horse has to have had a gray parent. This is so because in order to be gray, the horse has to have received a gray gene from a least one of its parents and any parent capable of passing a gray gene to is progeny would have been gray in coloration as the G gene would be expressed if the parent possessed it.

Examples of genes showing incomplete dominance are rare in horses but a well known one is the diluting gene which is symbolized by c^cr. It is the gene which when heterozygous with its normal allele (C⁺c^cr) is responsible for changing the sorrel coloration to palomino and when homozygous (c^crc^cr) for changing sorrel to cremello. This gene is incompletely dominant in that the heterozygous condition produces a coloration intermediate between those of the two homozygous conditions: sorrel + C⁺C⁺ = sorrel; sorrel + C⁺c^cr = palomino and sorrel + c^crc^cr = the nearly white cremello. Since cremello horses are unregisterable, it is important to understand how they can be produced so as to avoid such matings. This will be discussed further in the color inheritance section of this chapter.

Color Genetics

The inheritance of coat coloration and spotting patterns in horses has long been of great interest to breeders of horses. As a result, there is considerably more information available on the genetics of colorations than there is for the inheritance of genetic defects and quantitative traits such as speed or trainability. To most effectively discuss the inheritance of coat coloration in any species, it is useful to begin by establishing a standard of comparison by which to compare the effects of any genetic change (mutation) that differs from the standard. Usually this standard is called the wild type as it is the standard coloration of the species in its wild ancestors or relatives. An example of a wild type coloration is the tiger-striped or tabby pattern in the domestic cat. In the horse, it seems useful to use the bay coloration without any white spotting as the wild type or standard.

The bay coloration includes a combination of black hairs, generally on the mane, tail and lower legs and reddish colored hairs on the remainder of the body. There is considerable variation in the bay coloration from very light sandy bays to very dark, nearly black animals. This variation, however, will not be discussed further. Recessive mutant genes at the A and E loci are responsible for the changing of the bay or wild type coloration to the other two basic colorations, black and sorrel (or chestnut), respectively. A list of the common genes influencing coloration of horses is shown in Table 1.

Table 1. Common Color Mutants of Horses

Mutant Name/Description Symbol Pattern Relative to Wild Type^a

Black (will fade in intense sunlight) a Recessive

Sorrel (no black hairs in coat) e Recessive (but epistatic to the A locus)

Cremello (dilution of red or reddish colored hairs, homozygotes are nearly white) c^cr Incompletely dominant

Dilution of red, reddish or black hairs to dun, red dun or grulla D Dominant

Pangaré (dilution of sorrel to blond sorrel, black to seal brown) P Dominant

^aWild type = bay. Inheritance pattern is relative to the allele at the locus responsible for a bay coloration. Such alleles are indicated by the addition of a "+" to the symbol responsible for genes at a particular locus.

Red - Sorrel/Chestnut Coloration

The E locus

The sorrel (chestnut) coloration is produced when a horse is homozygous for the recessive gene symbolized by e. Thus, all sorrel horses are ee in genotype. Since horses must be homozygous in genotype (ee) in order to be sorrel, the so-called chestnut rule which states that chestnuts mated together produce only chestnuts becomes clear. Chestnuts must produce only chestnuts (reds) because they cannot carry any genes other than ee at this locus. Animals which are ee can produce eumelanin (black pigment) in their skin but not in their hair. The wild type allele at the E locus, which will be symbolized by E⁺ is needed to allow the production of eumelanin in the hair and thus bay horses (with both black and red hairs) and black horses have at least one E⁺allele and are E⁺E⁺ or E⁺e in genotype. Animals of genotype, E⁺e, while bay or black in coloration depending on the genotype of the A locus, can produce sorrel foals with a probability of 25% if bred to other heterozygotes (E⁺e animals) or with a probability of 50% if mated to sorrel ee horses. (See Table 2 for examples of expected frequencies of progeny colors from various matings). There are two ways of determining whether a bay or black horse is carrying e: 1) If the animal has a sorrel sire or dam it will have an e because any progeny of a sorrel parent must have an e gene. 2) Horses with parents that were both bay can also carry e. Their genetic status is only clarified if they produce a sorrel offspring.

Table 2. Expected Progeny Coloration from Various Matings

Matings/Genotypes Expected Progeny

Phenotype/Genotypes

Matings/Genotypes Expected Progeny

Phenotypes/Genotypes

Sorrel × Sorrel

(ee-- × ee--)
100% sorrel

(ee--)
Bay × Bay

(E⁺eA⁺A⁺ × E⁺eA⁺A⁺)
75% Bay (E⁺E⁺A⁺A⁺ or E⁺eA⁺A⁺)

25% Sorrel (eeA⁺A⁺)

Bay × Bay

(E⁺E⁺A⁺A⁺ × E⁺eA⁺A⁺)
100% Bay

(E⁺E⁺A⁺A⁺ or E⁺eA⁺A⁺)
Bay × Sorrel

(E⁺E⁺A⁺A⁺ × eeA⁺A⁺)
100% Bay (E⁺eA⁺A⁺)

Bay × Sorrel

(E⁺eA⁺A⁺ × eeA⁺A⁺)
50% Bay (E⁺eA⁺A⁺)

50% Sorrel (eeA⁺A⁺)
Black × Black

(aaE⁺E⁺ × aaE⁺E⁺)
100% Black (aaE⁺E⁺)

Black × Black

(aaE⁺e × aaE⁺e)
75% Black (aaE⁺E⁺ or aaE⁺e)

25% Sorrel (aaee)
Sorrel × Bay

(aaee × A⁺aE⁺e)
25% Bay (A⁺aE⁺e)

25% Black (aaE⁺e)

50% Sorrel (A⁺aee or aaee)

Sorrel × Bay

(aaee × A⁺aE⁺E⁺)
50% Black (aaE⁺e)

50% Bay (A⁺eE⁺e
Sorrel × Bay

(aaee × A⁺A⁺E⁺E⁺)
100% Bay (A⁺aE⁺e)

Black × Sorrel

(aaE⁺e × A⁺A⁺ee)
50% Bay (A⁺aE⁺e)

50% Sorrel (A⁺aee)
Black × Sorrel

(aaE⁺E⁺ × A⁺A⁺ee)
100% Bay (E⁺eA⁺A⁺)

Black × Sorrel

(aaE⁺e × A⁺aee)
25% Bay (A⁺aE⁺e)

25% Black (aaE⁺e)

50% Sorrel (A⁺aee or aaee)
Bay × Palomino

(A⁺A⁺E⁺C⁺C⁺ × A⁺A⁺eeC⁺c^cr)
50% Bay (A⁺A⁺E⁺eC⁺C⁺)

50% Buckskin (A⁺A⁺E⁺eC⁺c^cr)

Bay × Palomino

(A⁺A⁺E⁺eC⁺C⁺ × A⁺A⁺eeC⁺c^cr)
25% Bay (A⁺A⁺E⁺eC⁺C⁺)

25% Buckskin (A⁺A⁺E⁺eC⁺c^cr)

25% Palomino (A⁺A⁺eeC⁺c^cr)

25% Sorrel (A⁺A⁺eeC⁺C⁺)
Sorrel × Palomino

(A⁺A⁺eeC⁺C⁺ × A⁺A⁺eeC⁺c^cr)
50% Palomino (A⁺A⁺eeC⁺c^cr)

50% Sorrel (A⁺A⁺eeC⁺C⁺)

Black × Palomino

(aaE⁺E⁺C⁺C × A⁺A⁺eeC⁺c^cr)
50% Bay (A⁺aE⁺eC⁺C⁺)

50% Buckskin (A⁺aE⁺eC⁺c^cr)
Black × Palomino

(aaE⁺E⁺C⁺C⁺ × A⁺aeeC⁺c^cr)
25% Bay (A⁺aE⁺eC⁺C⁺)

25% Buckskin(A⁺aE⁺eC⁺c^cr)

50% Black (aaE⁺eC⁺C⁺ or aaE⁺eC⁺c^cr)

Black × Palomino

(aaE⁺eC⁺C⁺ × A⁺aeeC⁺c^cr)
12.5% Bay (A⁺aE⁺eC⁺C⁺)

12.5% Buckskin (A⁺aE⁺eC⁺c^cr)

25% Black (aaEeC⁺C⁺ or aaEeC⁺c^cr)

25% Palomino (A⁺aeeC⁺c^cr or aaeeC⁺c^cr)

25% Sorrel (A⁺aeeC⁺C⁺ or aaeeC⁺C⁺)
Palomino × Palomino

(A⁺A⁺eeC⁺c^cr × A⁺A⁺eeC⁺c^cr)
25% Sorrel (A⁺A⁺eeC⁺C⁺)

50% Palomino (A⁺A⁺eeC⁺c^cr)

25% Cremello (A⁺A⁺eec^crc^cr)

Palomino × Buckskin

(A⁺A⁺eeC⁺c^cr × A⁺A⁺E⁺eC⁺c^cr)
12.5% Bay (A⁺A⁺E⁺eC⁺C⁺)

12.5% Sorrel (A⁺A⁺eeC⁺C⁺)

12.5% Perlino (A⁺A⁺E⁺ec^crc^cr)

12.5% Cremello (A⁺A⁺eec^crc^cr)

25% Buckskin (A⁺A⁺E⁺eC⁺c^cr)

25% Palomino (A⁺A⁺eeC⁺c^cr)
Dun × Bay

(A⁺A⁺E⁺E⁺Dd⁺ × A⁺A⁺E⁺E⁺d⁺d⁺)
50% Dun (A⁺A⁺E⁺E⁺Dd⁺)

50% Bay (A⁺A⁺E⁺E⁺d⁺d⁺)

Table 2. Expected Progeny Coloration from Various Matings (con't)

Matings/Genotypes Expected Progeny

Phenotype/Genotypes

Matings/Genotypes Expected Progeny

Phenotypes/Genotypes

Dun × Bay

(A⁺A⁺E⁺eDd⁺ × A⁺A⁺E⁺ed⁺d⁺)
37.5% Dun (A⁺A⁺E⁺E⁺Dd⁺

or A⁺A⁺E⁺eDd⁺)

37.5% Bay (A⁺A⁺E⁺E⁺d⁺d⁺

or A⁺A⁺E⁺ed⁺d⁺)

12.5% Red Dun (A⁺A⁺eeDd⁺)

12.5% Sorrel (A⁺A⁺eed⁺d⁺)
Red Dun × Bay

(A⁺A⁺eeDd⁺ × A⁺A⁺E⁺ed⁺d⁺)
25% Dun (A⁺A⁺E⁺eDd⁺)

25% Bay (A⁺A⁺E⁺ed⁺d⁺)

25% Red Dun (A⁺A⁺eeDd⁺)

25% Sorrel (A⁺A⁺eed⁺d⁺)

Red Dun × Bay

(A⁺A⁺eeDD × A⁺A⁺E⁺E⁺d⁺d⁺)
100% Dun (A⁺A⁺E⁺eDd⁺) Red Dun × Red Dun

(A⁺A⁺eeDD × A⁺A⁺eeDD)
100% Red Dun (A⁺A⁺eeDD)

Red Dun × Black

(A⁺A⁺eeDD × aaE⁺E⁺d⁺d⁺)
100% Dun (A⁺aE⁺eDd⁺) Red Dun × Black

(A⁺aeeDD × aaE⁺E⁺d⁺d⁺)
50% Dun (A⁺aE⁺eDd⁺)

50% Grulla (aaE⁺eDd⁺)

Red Dun × Grulla

(A⁺aeeDd⁺ × aaE⁺eDd)
6.25% Black (aaE⁺ed⁺d⁺)

18.75% Grulla (aaE⁺eDD or aaE⁺eDd⁺)

37.5% Red Dun (A⁺aeeDD or A⁺aeeDd⁺)

12.5% Sorrel(A⁺aeed⁺d⁺ or aaeed⁺d⁺)

18.75% Dun (A⁺aE⁺eD⁺D⁺ or A⁺aE⁺eDd⁺)

6.25% Bay (A⁺aE⁺ed⁺d⁺)
Bay × Grulla

(A⁺aE⁺ed⁺d⁺ × aaE⁺eDd⁺)
18.75% Dun (A⁺aE⁺eDd⁺)

12.5% Red Dun (A⁺aeeDd⁺ or aaeeDd⁺)

18.75% Bay (A⁺aE⁺ed⁺d⁺)

18.75% Grulla (aaE⁺eDd⁺)

18.75% Black (aaE⁺ed⁺d⁺)

12.5% Sorrel (A⁺aeed⁺d⁺ or aaeed⁺d⁺)

Grulla × Grulla

(aaE⁺eDd⁺ × aaE⁺eDd⁺)
56.25% Grulla (aaE⁺E⁺DD or

aaE⁺E⁺Dd⁺ or aaE⁺eDD or aaE⁺eDd⁺)

18.75% Black (aaE⁺E⁺d⁺d⁺ or aaE⁺ed⁺d⁺)

18.75% Red Dun (aaeeDD or aaeeDd⁺)

6.25% Sorrel (aaeed⁺d⁺)
Black × Grulla

(aaE⁺E⁺d⁺d⁺ × aaE⁺E⁺Dd⁺)
50% Black (aaE⁺E⁺d⁺d⁺)

50% Grulla (aaE⁺E⁺Dd⁺)

Dun × Buckskin

(A⁺A⁺E⁺eC⁺c^crd⁺d⁺ × A⁺A⁺E⁺eC⁺C⁺Dd⁺)
18.75% Dun (A⁺A⁺E⁺_C⁺C⁺Dd⁺)

18.75% Bay (A⁺A⁺E⁺_C⁺C⁺d⁺d⁺)

18.75% Buckskin (A⁺A⁺E⁺_C⁺c^crDd⁺)

(with dun markings)

18.75% Buckskin (A⁺A⁺E⁺_C⁺c^crd⁺d⁺)

6.25% Red Dun (A⁺A⁺eeC⁺C⁺Dd⁺)

6.25% Sorrel (A⁺A⁺eeC⁺C⁺d⁺d⁺⁾

6.25% Palomino (A⁺A⁺eeC⁺c^crd⁺d⁺)

(with dun markings)

6.25% Palomino (A⁺A⁺eeC⁺c^crd⁺d⁺)
Buckskin (with dun markings)

(A⁺A⁺E⁺eC⁺c^crDd⁺)

×

Sorrel (A⁺A⁺eeC⁺C⁺d⁺d⁺)
12.5% Dun (A⁺A⁺E⁺ec⁺C⁺Dd⁺)

12.5% Bay (A⁺A⁺E⁺eC⁺C⁺d⁺d⁺)

12.5% Buckskin (A⁺A⁺E⁺eC⁺c^crDd

(with dun markings)

12.5% Buckskin (A⁺A⁺E⁺eC⁺c^crd⁺d⁺)

12.5% Red Dun (A⁺A⁺eeC⁺C⁺Dd⁺)

12.5% Palomino (A⁺A⁺eeC⁺C⁺d⁺d⁺)

(with dun markings)

12.5% Sorrel (A⁺A⁺eeC⁺C⁺d⁺d⁺)

12.5% Palomino (A⁺A⁺eeC⁺c^crd⁺d⁺)

Black

The A locus

Most black horses are the result of being homozygous for a recessive gene at the A locus which is symbolized by a. Thus, most black horses are aa in genotype. The possibility of a dominant black gene at the E locus (E^d) has been discussed and may be responsible for the black coloration in Clydesdales, but this gene (if it exists) does not appear to be common. The wild-type allele at the A locus, symbolized as A⁺, allows expression of black only in the mane, tail and lower legs, whereas, the recessive gene allows the production of black pigment over the entire body. For any black pigment to be produced anywhere on the body, however, the E⁺ gene must be present at the E locus. Thus, bay horses are A⁺_E⁺_ in genotype and black horses are aaE⁺_ in genotype. All horses that are ee at the E locus will be sorrel regardless of their genetic makeup at the A locus. In fact, the only way to determine the genotype of a sorrel horse at the A locus is through its progeny or by the coloration of its parents or both. For example, a sorrel horse would be known to carry at least one a allele if it either produces a black progeny or had a sire or dam that was black. The presence of at least one A⁺ allele in a sorrel horse could be ascertained if such a horse produced a bay progeny from a black mate. A sorrel could be produced from the mating of two black animals both of genotype aaE⁺e (probability, 25%). A sorrel produced from such a mating would have to be aaee in genotype. In many cases, however, it may not be possible to determine with accuracy the genotype at the A locus of a sorrel horse.

The so-called chestnut rule has already been discussed. A similar black rule would be that blacks mated together do not produce bays. As mentioned above, blacks mated together could produce sorrels, but never bay as this would require the presence of the A⁺ gene which black animals do not have. Exceptions to this have occurred in Clydesdales which indicates an alternative means of producing a black coloration. Since bay horses are A⁺_E⁺_ in genotype, they may possess a, e or both and thus, bays mated to bays may produce bays, blacks or sorrels depending on whether or not they carry either recessive gene. If bay horses are mated that are both A⁺aE⁺e in genotype the probabilities of the colorations of resulting foals are 9/16 bay (A⁺_E⁺_), 3/16 black (aaE⁺_) and 4/16 sorrel (__ee). Blacks (aaE⁺) mated to bays (A⁺_E⁺_) can also produce all three colors depending on the recessive genes carried, as could sorrels (__ee) mated to bays. Similarly, blacks mated to sorrels could produce bays (if the red carried A⁺) or sorrel (if the black carried e). Clearly predicting the coloration of an expected foal cannot often be made with a high degree of accuracy except in certain circumstances. If the genotypes of the parents are known, however, one can determine what colorations are possible and their probabilities of occurrence. Examples of many combinations of parental genotypes and expected progeny phenotypes are shown in Table 2.

The Dilute Colorations

The three basic colorations, bay, black, and red may be diluted to lighter colorations by the dilution genes. In recent years, it has been shown convincingly that two different genes are responsible for producing dilute colorations. One gene, symbolized by c^cr, shows incomplete dominance and is responsible for producing the palomino and buckskin colorations and the other, symbolized by D, shows complete dominance and is responsible for the dun, red dun and grulla colorations. A third gene, symbolized by P for the Pangaré effect, is dominant in mode of inheritance and its presence in an animal results in the lightening of the muzzle, areas over the eyes, the flanks and the inside of the legs. The blond sorrel coloration in Belgians is an example of the P gene acting on sorrel. Finally, the silver dapple gene, symbolized by Z and fairly common in ponies but not in horses, results in a lightening of eumelanin (black pigment) wherever it appears on the horse.

Palomino and Buckskin Colorations

The gene responsible for the buckskin and palomino colorations, c^cr, acts upon red pigment (phaeomelanin) to produce a lightened color, generally some shade of yellow when in the heterozygous state in conjunction with its wild-type allele (C⁺c^cr), and produces a very light yellow, nearly white coloration when homozygous (c^crc^cr). Thus, horses that are genetically sorrel (with C⁺C⁺) become palomino when the horse is C⁺c^cr and cremello (nearly white) when the horse is homozygous for c^cr (c^crc^cr). Similarly, the reddish areas of bay horses are diluted to yellow and thus, are buckskins if the bay is heterozygous of c^cr (C⁺c^cr). When a bay horse is homozygous for c^cr, the color produced is called Perlino, also a nearly white coloration.

Since the palomino and buckskin colorations both are caused by a heterozygous genotype at the C locus (C⁺c^cr) it should be clear why neither of these colors breed true; that is, no palomino bred to another palomino will produce all palomino progeny. The expected result of palomino to palomino matings (eeC⁺c^cr × eeC⁺c^cr) is 25% cremello (eec^cr):50% palomino (eeC⁺c^cr): and 25% sorrel (eeC⁺C⁺). Since the cremello coloration is considered undesirable and generally is not registerable, palominos more commonly are produced by matings of palominos to sorrels resulting in 50% palomino progeny (the same proportion as the palomino to palomino matings are expected to produce) and 50% sorrel progeny. A simple procedure to produce 100% palomino progeny from a cross would be to select a very superior cremello stallion (eec^crc^cr) and breed him to sorrel mares (eeC⁺C⁺) which should produce all palomino progeny (eeC⁺c^cr).

The mating of a palomino (eeC⁺c^cr) to a bay of genotype E⁺E⁺C⁺C⁺ would be expected to produce 50% buckskin (E⁺eC⁺c^cr) progeny. If the bay that is mated to the palomino is heterozygous for the sorrel allele (E⁺eC⁺C⁺), four types of progeny could be expected: 25% bay (E⁺eC⁺C⁺), 25% buckskin (eeC⁺c^cr), 25% sorrel (eeC⁺C⁺) and 25% palomino (eeC⁺c^cr). The exact same ratio of progeny colorations would be expected from the mating of a buckskin known to carry e (as a result of producing a sorrel or palomino foal previously or having a parent of either of these colorations) and a sorrel. Buckskins bred to bays will produce 50% bay and 50% buckskin progeny if both parents are not heterozygous at either the A or E loci. If both the buckskin and the bay are heterozygous for the sorrel gene (E⁺eC⁺c^cr-buckskin × E⁺eC⁺C⁺-bay), the expected colorations of their progeny would be 37.5% bay, 37.5% buckskin, 12.5% sorrel and 12.5% palomino. Buckskins carrying e that are mated together would be expected to produce 3/16 Perlino, 3/8 buckskin, 3/16 bay, 1/16 cremello, 1/8 palomino and 1/16 sorrel. In actual practice of mating a single pair of buckskins of this type over several years, every foal born could be a different color.

The c^cr gene, however, has little or no effect on the black coloration. A black horse (aaE⁺_) can be C⁺c^cr in genotype and still be black. Some, however, may be a smokey black. Thus, it is possible for a black horse to be mated to a sorrel and result in a palomino or a buckskin progeny. If the black horse was a genotype aaE⁺eC⁺c^cr and the sorrel AAeeC⁺C⁺, the expectations in the progeny resulting would be 1/4 bay, 1/4 sorrel, 1/4 palomino and 1/4 buckskin. If the sorrel also was heterozygous for a, then the expected progeny would be 1/4 black (aaE⁺eC⁺c^cr and aaE⁺eC⁺C⁺), 1/4 palomino (A⁺aeeC⁺c^cr and aaeeC⁺c^cr), 1/8 bay (A⁺aE⁺eC⁺C⁺) and 1/8 buckskin (A⁺aE⁺eC⁺c^cr). It would be impossible to determine without progeny testing (i.e., mating to sorrel) whether or not the black animals produced from this last mating carried c^cr. Only the production of a palomino or buckskin progeny from the mating of a black animal to a bay or sorrel is proof that the black animal carries c^cr. The only exception to this would be the hypothetical case where a black animal had a Perlino or cremello parent and would thus, have to carry at least one c^cr gene. Other example of matings involving the c^cr gene and expected phenotypes of the progeny are shown in Table 2.

The Dun Dilution

A dun gene, which is symbolized by D, is completely dominant to its normal, non-diluting wild-type allele, d⁺. The D gene, unlike c^cr, acts on both eumelanin and phaeomelanin and is responsible for producing the dun coloration from bay or sorrel and the grulla or mouse coloration from black. The dun coloration may be similar to the buckskin but red duns are generally quite distinctly different from palominos, being more of a light red, apricot or flesh color than the generally yellow coloration of the buckskin and also by having darker points, head, feet, mane and tail than the palomino which generally has a uniform coloration throughout the body with a lighter mane and tail. Also, the red dun coloration is characterized by dun factor markings, a dark red dorsal stripe and often by leg barring (zebra striping) and shoulder striping or shadowing.

In general, the dun coloration (the result of the action of the D gene on bay) is a darker, duller coloration than the buckskin coloration and has the dun factor markings described for the red dun. All, duns will not have the white hairs (frosting) that may be present in the mane and tail of a buckskin horse. The problem of determining whether a horse is a dun or a buckskin may be aggravated by the fact that buckskins and duns may be mated together and thus, genetically bay horses carrying both D_ and C⁺c^cr may be produced. A horse of this genotype may appear as a buckskin with a dorsal stripe. The production of both red dun and palomino foals from such an animal when mated to sorrel horses would be considered proof of such a genotype.

It would be possible to produce duns, red duns or produce which breed true if all such animals were DD in genotype. If duns which were homozygous for D and heterozygous for a and e (A⁺aDDE⁺e) were mated together, they would produce duns, produce and red duns with a probability of 9/16:3/16:4/16. A homozygous dun stallion of the above genotype would produce half dun and half red dun progeny from sorrel mares (A⁺A⁺ee) and half dun and half grulla foals when mated to black mares (aaE⁺E⁺). A red dun which is heterozygous for D (A⁺A⁺Dd⁺ee⁺), if bred to bays of genotype A⁺A⁺d⁺d⁺E⁺e would be expected to produce 25% dun (A⁺A⁺Dd⁺E⁺e), 25% red dun (A⁺A⁺Dd⁺ee), 25% bay (A⁺A⁺d⁺d⁺E⁺e) and 25% sorrel (A⁺A⁺d⁺d⁺ee) foals. More examples of expected progeny phenotypes from mating involving the dun gene are shown in Table 2.

The Pangaré Dilution

The Pangaré type of color dilution also seems to dilute both red and black pigment like D, but not in a uniform fashion. The coloration is most diluted on the muzzle, over the eyes, in the flanks and on the insides of the legs. Perhaps the best example of this coloration is in Belgians where blond sorrels are the result of the action of P on sorrel. The seal brown coloration is thought to be produced by the P gene acting on black. The production of a palomino from the mating of a seal brown mare to a sorrel stallion can be explained using this explanation for the seal brown coloration. The seal brown mare which produced the palomino was thus, aaE⁺eC⁺c^crP_ in genotype and its palomino offspring, eeC⁺c^crP?_ in genotype.

The Silver Dapple Gene

One final gene which dilutes pigment is called the silver dapple gene and is found primarily in pony breeds. This gene is symbolized by a Z. The action of the silver dapple gene is to dilute eumelanin (black) pigment and thus, can produce lightened bays with dappling and essentially cream or off-white manes and tails. The Z gene apparently has no effect on phaeomelanin (red) pigment.

Other colorations occur in horses, such a liver chestnuts, that are not clearly understood genetically. Some liver chestnuts may be due to the action of a recessive gene symbolized by b in other mammalian species. This gene when homozygous, results in the production of brown (chocolate) pigment where black pigment would have been produced (if the animal was B⁺_ instead of bb in genotype). Thus, many liver chestnuts may be aabbE⁺_ in genotype. Other colorations are likely due to combinations of the actions of several of the genes already discussed. For a discussion of many of these, please refer to the book, Horse Color, by Sponenberg and Beaver.

White Spotting Mutants

There are a number of mutants which produce areas of white spotting on horses. Some of these mutants are well understood while others are not. A listing of white spotting mutants commonly seen in horses is shown in Table 3. Certain white spotting or roaning mutants are lethal when homozygous whereas others, which may appear very similar, are not. Also, since certain white spotting patterns are required for registry by several associations (Paint, Appaloosa, etc) and white spotting of the feed and head may be considered desirable in other breeds, an understanding of the genetic mechanisms involved is useful.

Table 3. Common White Spotting Mutants of Horses

Mutant Name/Description Symbol Pattern Relative to Wild Type^a

Tobiano paint T Dominant

Overo paint O Dominant

Sabino spotting (wide blaze, white stocking and underline often with roaning in the flanks) Sb Incomplete dominant

Roan R Dominant, homozygous lethal

White W Dominant, homozygous lethal

Leopard/blanketed Lp Incomplete dominant

^aWild-type = Solidly pigmented, no spotting. The inheritance pattern is relative to the allele at the same locus responsible for the wild type pattern. Wild type alleles at a particular locus are indicated by the addition of a "⁺" to the symbol indicating the locus.

Gray

The gray coloration of horses is not usually observable at birth but becomes increasingly evident as the horse ages and more and more of the pigmented hairs are replaced with white ones. Often aged, gray animals will appear white, such as in the Lippizaners. A dominant gene, symbolized by G, is responsible for the gray coloration. Apparently, the gene is completely dominant meaning the Gg (homozygous) horses cannot be distinguished from Gg⁺ (heterozygous) horses. Since the gene is dominant, all gray horses must have at least one gray parent in order to be gray. The horse has to have received a G gene from at least one parent and that parent would have been gray if it possesses G gene.

The gray color does not appear to be linked to any color gene and thus any color can turn gray due to the action of the G gene. In older horses, it may be impossible to determine what coloration the horse was prior to becoming nearly white.

A gray horse which is GG in genotype will produce only gray progeny. Production of only gray foals from seven or more non-gray mares is strong evidence that a stallion is homozygous for the G gene. It does appear that other genes are responsible for the rate at which the graying process occurs as some breeds gray out faster than others. Also, some grays dapple while others do not. The dappling pattern is thus, likely due to a separate gene or genes as well.

Roan

The roan coloration differs from the gray in that the mixture of white and pigmented hairs of the roan horse is present at birth and does not change throughout its lifetime. The genetic control of the roan color has been somewhat unclear in the past due to the fact that a number of genes produce a pattern that is called roan.

Probably the most common roan pattern is produced by a gene symbolized by R. This gene which is found in Belgians and Quarter Horses produces a uniform roan pattern over the body except that the head and lower legs are solidly pigmented. The R gene is lethal when homozygous so that horses homozygous for R die early in gestation and apparently are reabsorbed. If roan horses are mated together, the progeny which are born will be two-thirds roan and one-third non-roan (bay, sorrel, black or whatever color their genotype at other loci determines). The reason for this ration of foals born is that the original 1/4 RR:2/4Rr⁺:1/4r⁺r⁺ ration is modified by the early death of the RR embryos, thus leaving only the Rr⁺ and r⁺r⁺ foals to be born which will occur in a 2:1 ration. Therefore, to prevent a lowering of the probability of a birth of a live foal, roan mares should not be bred to roan stallions. Roan animals, like gray ones, will always have a roan parent, except in the very unlikely event of a mutation, due to the dominance of a roan gene. Since the homozygous condition (RR) is lethal in utero, all roans are Rr⁺ in genotype and, therefore, would be expected to produce 50% roan foals when bred to non-roan (r⁺r⁺) mares.

An interesting fact about the R gene is that it is linked to (on the same chromosome as) the E⁺/e (chestnut/sorrel) locus. This is perhaps most easily explained by discussing the data reported by Sponenberg et al. (1984) which documented the linkage. A bay roan, Brabant Belgian stallion produced a total of 57 foals from chestnut American Belgian mares. Thirty of the foals were by roan, 25 were chestnut, one was a bay and one was a chestnut roan. If these genes were not linked, equal quantities of each of the four colorations would have been expected. The progeny of this bay roan stallion indicate that the genetic arrangement of his genes on a particular pair of chromosomes where the E⁺ gene is on the same chromosome as the R gene and the e gene is on the same chromosome as the r⁺ gene. Since the chestnuts and the bay roans were the desired colorations in this case, the linkage was helpful. If, however, chestnut roans were desired, the linkage situation in this sire would be frustrating.

A complicating factor regarding the inheritance of the roan coloration in that at least three other genes also produce colorations that may be called roan. Sponenberg and Kilby (1987) describe a coloration that they call frosty which occurs in Belgians and is very similar to that caused by the R gene. A means of differentiating this coloration from that produced by the R gene is that frosty roans show roaning over a greater part of the body, including the head and legs whereas the head, feet and legs are not roaned on roans produced by the R (homozygous lethal) gene.

A spotting pattern which occurs in a number of breeds including the Clydesdale, Quarter Horse, Dutch Warm Bloods and perhaps other breeds has been called sabino and is generally characterized by areas of white on the underline and feet along with usually an extensive blaze and often with some roaning, at least in the flank region. The amount of roaning varies from just a little in the flank region to extensive areas of roaning on the sides. The roaning associated with the sabino gene, however, can be distinguished from that produced by R due to its lack of uniformity. Sabino-type roan patterns include patches of white interspersed with the roaned areas and these white and roaned areas extend upward from the belly. The clydesdales which are referred to as roans actually carry the sabino gene, symbolized by Sb instead of the R gene. The Sb gene is dominant in mode of inheritance. Horses which are homozygous for Sb may have more white than is desired in many breeds so it is probably desirable to breed horses with sabino patterns to nonsabino (sb⁺sb⁺) horses, horses with only white socks and minimal amounts of white on the head or no white spotting at all.

A third roan-type pattern which is often referred to as roan is observed in Quarter Horses and is more appropriately called a rabicano pattern to distinguish it from the true roan, the roan produced by the R gene. The gene symbol for the rabicano gene in Rb. This type of roaning can also be distinguished from the true roan pattern because of its lack of uniformity over the entire body, usually being confined to the flanks and sides with some white areas sometimes being produced. The rabicano gene does not, however, produce extensive solid white areas on the face and legs like the Sb gene. Perhaps the best indicator that a horse carries Rb is a ring of white hairs (coon-tail) around the base of the tail. True roans, those produced by the action of R, do not have this white in their tails. The rabicano gene is dominant in mode of inheritance. The effect of homozygosity for Rb has not been reported.

Finally, other types of roan patterns occur in the Appaloosa breed with one coloration being known as a varnish roan. The exact mode of inheritance of such colorations is not known.

Paint Patterns

The paint or pinto patterns of horses are generally separated into two categories called tobiano and overo. In tobiano patterns, the white areas appear to originate from the dorsal parts of the body (neck and back) and the white areas are distinctly separated from pigmented areas. Overo patterns, on the other hand, appear to originate from the underside of the horse and there is an intermixing of pigmented and white hairs (a roan-like pattern) in the borders of pigmented and white areas of the horse. Often a considerable amount of white on the head is associated with the overo pattern.

The mode of inheritance of the two types of paint patterns are also different. The gene responsible for the tobiano type of paint is dominant in mode of inheritance and is symbolized by the symbol, T. The gene responsible for the overo pattern generally appears to be recessive in mode of inheritance as overos can be produced from parents which are both solid colored.

Since the gene responsible for the tobiano pattern is dominant in mode of inheritance, it can easily be produced by mating solid-colored mares to tobiano paint stallions. half of the progeny of heterozygous paint stallions (Tt⁺) mated to solid colored (t⁺t⁺) mares would be expected to be paints (T_) and all of the foals from solid mares and homozygous tabiano stallions (TT) should be paints (T_). One could effectively determine if a stallion is homozygous for T by the production of only paint progeny from sever or more solid-colored (T⁺t⁺) mares. The probability of the stallion being homozygous is at least .9922 if seven or more paint progeny from such matings are produced. Of course, any horse which is homozygous tobiano (TT) would have to have had both parents which were tobianos. It should also be emphasized that since the gene responsible for the tobiano pattern is dominant, tobianos mated to tobianos should be expected to occasionally produce solid colored (t⁺t⁺) progeny. The mating of Tt⁺ × Tt⁺ (heterozygous paints) together should produce 25% nonpaint (t⁺t⁺) progeny. Since some paints are homozygous, less than 25% nonpaint foals would be expected on a nationwide basis when tobiano paint stallions are mated to tobiano paint mares.

The Overo paint spotting pattern is caused by another dominant gene, O, which, when homozygous, results in a foal that is born solid white. Such white foals always die of colic within 48 hours of birth as a result of a lack of enervation of the gut. This fact should result in the avoidance of Overo to Overo matings except for the fact that some horses that might technically have sufficient white spotting to be classified as an Overo may not in fact carry the O gene. These horses might actually be white spotted due to the Sabino, and not the Overo gene. Now breeders of overo paint horses have the opportunity to use DNA testing available through the Veterinary Genetics Laboratory of the University of California-Davis to determine whether or not the overo paint pattern of their horse is, or is not, due to the O gene. Horses showing an overo spotting pattern but not carrying the O gene may safely be mated to horses that are overo due to the O gene as no homozygous O progeny will result.

Solid white horses do exist in several breeds, such as the American Albino and the Tennessee Walking Horse, which are born white and are fully healthy. Such horses have dark eyes which helps distinguish them from horses with the nearly white cremello or perlino colorations which generally have blue eyes. When such dark-eyed white horses are mated together, the progeny born would be two-thirds white and one-third colored (bay, sorrel, black, etc.). This ratio is produced as the homozygotes for the gene responsible for this white coloration (W) die early in pregnancy so that only the heterozygotes (Ww⁺) and the homozygous normal animals (w⁺w⁺), the colored ones, survive and are born. Breeders of this type of white horses will need to be satisfied with about a 255 lower foaling rate if they desire to use only white horses in their breeding programs. Perhaps, however, this can be overcome partially if mares conceiving WW embryos early in the breeding season return to estrus fast enough to rebreed later in the breeding season. The chance that a mare would conceive WW foals twice during a breeding season would be small (1 change in 16) so that the overall decrease in number of foals born would not likely be nearly as high as 25%.

The Spotting Patterns of Appaloosa Horses

The inheritance of the spotting patterns in the Appaloosa breed is not as clearly understood as those of most other breeds. One of the problems involved is that many different types of roaning or spotting patterns can be registered as Appaloosas. It is beyond the scope of this chapter to specifically describe all such variations. The reader is referred to Sponenberg and Beaver for more detailed discussion of these patterns. There is clear evidence of a simple mode of inheritance, however, for at least one type of Appaloosa pattern, the blanketed with spots and leopard patterns. A dominant gene which has been given the symbol, Lp, for the leopard pattern, is thought to control the blanket and leopard patterns. Horses which are heterozygous for Lp and its nonspotting allele lp⁺ are either blanketed with large spots or show the leopard pattern, again with large spots. The genetic factors responsible for determining the amount of white on heterozygous horses are likely quantitative in nature, that is a number of genes each with an individual small effect, influence whether a horse with the genotype Lplp⁺ has a small blanket, a blanket covering most of the rear half of the horse or a full leopard pattern. Horses that are homozygous for Lp would be expected to always be leopards, generally of the few spot variety. Stallions of this type are probably not highly prized due to their near white coloration but would be the most appropriate mate for mares with minimal or no Appaloosa markings if the goal is to produce Appaloosa-marked foals. The progeny from such matings should be Lplp⁺ in genotype which is likely to produce a well-marked Appaloosa. Examples of expected progeny phenotypes for white spotting from parents with various genotypes for white spotting are shown in Table 4.

Table 4. Expected Spotting Patterns of the Progeny from Various type of Matings

Matings/Genotypes Expected Progeny Phenotype/Genotypes

Tobiano Paint × Solid (nonspotted)

(Tt⁺ × t⁺t⁺)
50% Tobiano Paint (Tt⁺)

50% Solid (t⁺t⁺)

Tobiano Paint × Solid

(TT × t⁺t⁺)
100% Tobiano Paint (Tt⁺)

Overo Paint × Solid

(Oo⁺ × o⁺o⁺)
50%Overo Paint (Oo⁺)

50% Solid (o⁺o⁺)

Overo Paint × Overo Paint

(Oo⁺ × O⁺o)
25% Lethal White (OO)

50% Overo Paint (Oo⁺)

25% Solid (o⁺o⁺)

White × "Pigmented"

(Ww⁺ × w⁺w⁺)
50% White (Ww⁺)

50% "Pigmented" (w⁺w⁺)

White × White

(Ww⁺ × Ww⁺)
66.6% White (Ww⁺)

33.4%"Pigmented" (w⁺w⁺)

Roan^c × Solid

(Rr⁺ × r⁺r⁺)
50% Roan (Rr⁺)

50% Solid (r⁺r⁺)

Roan × Roan

(Rr⁺ × Rr⁺)
66.6% Roan (Rr⁺)^b

33.4% "Pigmented" (r⁺r⁺)

Gray × "nongray"

(GG⁺ × g⁺g⁺)
100% Gray (Gg⁺)

Gray × "nongray"

(Gg⁺ × g⁺g⁺)
50% Gray (Gg⁺)

50% "nongray" (g⁺g⁺)

Gray × Gray

(Gg⁺ × Gg⁺)
75% Gray (GG or Gg⁺)

25% "nongray" (g⁺g⁺)

Sabino (roan^d) × Solid

(Sbsb × sb⁺sb⁺)
50% Sabino (roan^d) (Sbsb⁺)

50% Solid (sb⁺sb⁺)

Sabino (roan^d) × Sabino

Sbsb⁺ × Sbsb⁺)
25% Excessively white sabino (SbSb)

50% Sabino (roan^d)(Sbsb⁺)

25% Solid (sb⁺sb⁺)

Blanketed × Blanketed

(Lplp⁺ × Lplp⁺)
25% Leopard^e (LpLp)

50% Blanketed^f (Lplp⁺)

25% Solid (lp⁺lp⁺)

^aWhite foals which will die within 48 hours of birth have been produced from such a cross.

^bOne-fourth of the foals conceived, those homozygous for white or roan, will die and be reabsorbed in utero.

^cRoaning in uniform over body except for the head and legs.

^dFlanks are roan, blaze faced.

^eMay be "few spots."

^fMay be a leopard with large spots.

Facial White Markings and Markings on the Feed and Legs

White markings on the face such as star, snip, strip, stripe, blaze, bald, etc., and white markings on the feet and legs varying from white just above the hoof to white which extends above the knee or hock have not been shown to be simply inherited. This is, they do not seem to be controlled by one gene or a small number of genes, but instead, seem to be quantitatively inherited, controlled by a relatively large number of genes. Woolf (1989) has recently analyzed the inheritance of white facial markings in the Arabian horse and his results support the conclusion of quantitative genetic control over white facial markings. As a consequence of this quantitative mode of inheritance, it is difficult to control the quantity of white on the face and legs. In general, matings of stallions with fairly extensive white markings to similarly marked mares will produce foals with substantial amounts of white. Similarly, stallions with no white or perhaps just a snip or one white sock will tend to produce minimally marked foals when bred to mares without much white.

Inheritance of Genetic Defects

***The number of congenital defects which are clearly understood to be genetically controlled is not great as compared to those of cattle. This does not mean that horses necessarily carry fewer defective genes, just that sufficient data have not been available to determine whether or not certain kinds of deformities or diseases are genetically controlled. Most genetically controlled defective conditions are caused by genes which are recessive in mode of inheritance. The reason for this is dominant genes which are cause lethal conditions or defects are either self-eliminating (as in the case of the lethal ones which kill all animals possessing the gene) or are easily seen in all animals which possess the gene and thus, such animals are not used for breeding, which also eliminates the gene. An example of a dominant gene which produces a defect in Belgians was discussed by Trommershausen-Smith (1980). The condition produces foals which are born lacking the iris in both eyes and later development of cataracts made most of the affected animals blind. The affected animals were all the progeny of a single stallion who produced 65 progeny with the condition and 78 normal progeny. Thus, it appeared that a mutation had occurred which produced a dominant gene which was responsible for this defect. In such a case, when the mutant gene involved id dominant, its removal is relatively easy if only the normal horses are used for breeding. The normal progeny of this horse will not carry the mutant gene because if they did, they would show it.

The situation regarding elimination of the much more common, recessive mutant genes is much more complex. Perhaps the most frequently discussed recessive gene is the gene responsible for the combined immunodeficiency (CID) condition in Arabian horses. Since the gene responsible for this condition is recessive, the carrier (heterozygous) horses are perfectly healthy. The method for determining that a horse is a carrier of CID is through their production of an affected offspring (homozygous for the recessive cid gene) when mated to another carrier. Even in the case of carriers being mated to carriers, however, there is only a 25% chance that any given foal will be born with the CID condition. Thus, a stallion that carries the CID gene could produce many progeny without producing any abnormal progeny, especially when he is mated to nearly all homozygous normal (noncarrier) mares. In fact, it is probably through the extensive use of some outstanding stallion, which was a carrier of the CID gene, and those of his descendants which were also carriers, that the gene became a problem within the Arabian breed. While the heterozygous (carrier) state of such a stallion was almost certainly unknown to his owners, some stallion currently at stud may be known to be carriers by their owners as a result of having produced an affected progeny. If for financial or other reasons, owners of such stallions (or mares which are also known carriers) fail to disclose this information and continue to use the animals for breeding (especially the public use of a stallion at stud), the result can be serious harm done to the breed through increasing the frequency of the incidence of CID.

Even though the situation is quite clear regarding the use of the known carrier (unless extremely superior to the available breeding animals, they shouldn't be used), it is somewhat less clear regarding a son or grandson of a horse which has become known to be a carrier. A son of a known carrier has a 50% chance of being a carrier and a grandson, a 25% chance. Unfortunately, there is no method of testing a young stallion for the presence of the CID gene except through matings to mares known to be carriers themselves. If a stallion produces 16 normal progeny from mares known to be carriers, there is only a very remote chance (less than 1 in 100) that he is a carrier. Of course, the production of one or more foals affected with CID confirms that he is a carrier. Unfortunately, it would be difficult to assemble enough mare that are known carriers to produce 16 progeny in a short period of time. The use of embryo transfer so that the known carrier mares could produce multiple offspring during a year's time could speed the process.

A sire can be tested for the presence of the CID gene as well as any other recessive gene that he may carry by mating him to his daughters. Unfortunately, it takes an enormously large number of such matings, 35 if each daughter produces one foal; or 38 if each of 19 daughters produces 2 foals. Thus, it is unlikely that this procedure will be used often.

Other than CID only a few other defective conditions in the horse have been clearly documented to be controlled by recessive genes. On condition, epitheliogenesis imprefecta, causes the skin to be missing hair on the legs and other parts of the body resulting in infections which have always killed the foals within a few days of birth. This condition was shown rather clearly to be controlled by a recessive gene. It does not, however, appear to be a problem in any breed in the U.S. today.

Another report discussed a leg defect in shetland ponies which affected the ponies' locomotion (Trommershausen-Smith, 1987). Eight matings of affected animals to other affected animals produced only affected progeny. The same mares bred to normal stallions produced only normal progeny. This clearly indicates that a recessive gene is responsible for this leg defect. While it is not known whether or not this gene is present at a high enough frequency to be a problem in the shetland breed, it very likely is still present. Testing procedures for recessive genes where the homozygous (affected) animals are alive and fertile are simpler in that only seven normal progeny are required from a sire being tested for a recessive gene when he is mated to homozygous mares.

Neonatal isoerythrolysis (NI) is another condition which is genetically controlled which may result in the death of a foal. The condition is caused by destruction of the foal's red blood cells by antibodies from the dam's colostrum. The condition is similar to the Rh system, blood group incompatibility in humans. The mare may produce antibodies to the foal's red blood cells if she lacks certain blood antigens (proteins) that the foal possesses. Two such blood antigens are referred to as Aa and Qa. When a mare that is Qa-negative (does not possess the Qa-antigen) is bred to a Qa-positive stallion, the foal will have a 50 or 100% chance (depending on whether the stallion is heterozygous or homozygous for Qa) of having Qa-positive. If the Qa-negative dam of this foal has become sensitized to Qa by a previous pregnancy in which the fetus was Qa positive or perhaps by a transfusion with Qa-positive blood, she may produce anti-Qa antibodies in her colostrum, which when ingested by the foal attack its red blood cells. The result of the red blood cell destruction may be a severe anemia or even death of the foal.

To avoid such problems, two procedures have been suggested. If a mare has previously produced foals with NI, it may be possible to determine what blood antigen she is sensitized to (Aa or Qa, for example) and then try to locate a stallion which does not carry that particular antigen. Another option is to wait until about 3 weeks before the foal id due and then test the mare's serum for the presence of antibodies which could attack the red blood cells the foal is likely to carry. If the foal is likely to be affected by NI, then it is recommended that the foal be fed colostrum from a mare not likely to be producing antibodies to the foal's red blood cells. After 36 hours it is reportedly safe for the foal to nurse its own dam.

Blood Typing of Horses

Most associations which register horses require that the blood types of stallions be on file with the association. The Jockey Club also requires that Thoroughbred mares and their foals be blood typed prior to registration. In addition, several breed associations select foals at random to blood type to verify parentage.

Blood samples are evaluated for blood types (red blood cell antigen type) at seven blood group loci: A, C, D, K, P, Q, and U. In addition, variation is evaluated for several enzymes and proteins of the book including albumin, transferrin, postalbumin, prealbumin, esterase, protease inhibitors and others. Tables 5 and 6 give a partial listing of the alleles formed at various red blood group, enzyme and protein loci in horses.

Blood typing of horses is used primarily for parentage testing. Blood typing cannot prove that a horse is the sire of a foal without any possibility of error; it can, however, prove that a horse could not have been the sire. There are eight loci which are responsible for eight blood group systems in horses. Within each of these systems are alleles which are responsible for producing many different blood types (Tables 5 and 6). For example, at the P locus there are three alleles symbolized by p^a, p^b and p. The P allele is recessive to all the others, which, in turn are codominant to each other. By codominant, it is meant that an animal which carries the alleles p^aand p^b will have the phenotype (blood type) A/B. Both the A and the B antigens are formed on the surface of the red blood cells, are expressed and can be tested for with appropriate antibodies. These antibodies are produced by transfusing the blood of one animal carrying a particular blood factor (A¹, for example) into a horse without that particular factor, say a/a, in genotype. The a/a horse responds to the transfusion by producing antibodies against A¹ which are then extracted from its blood to produce what is called a reagent. when red blood cells of a horse that carries the A¹ antigen are mixed with the A¹ reagent, the antibodies in the reagent attack the red blood cells and hemolysis (breakage of the red blood cells) or clumping of the red blood cells occurs.

Table 5. Alleles of blood group loci in horses^a

Locus

A C D K P Q U

adf a ad a a abc a

a __* dk __* b ac __*

b d __* b

c dgh c

e de __*

bc dek

be dfk

ce bc

bce cg

__* cegi

cefg

cf

^aAdapted from Bowling and Clark (1985).

^*The -- indicates that an allele is present at that locus which does not produce an antigen that can be detected.

Table 6. Alleles of selected serum and red blood cell protein markers in horses^a

Locus

Albumin Transferrin Postalbumin Prealbumin Esterase

A D F 2 F

B F₁ S 3 G

F₂ 4 H

H₁ 5 I

H₂ 6

O 7

R

^aAdapted from Bowling and Clark (1985) and Trommershausen-Smith et al. (1976).

The basic principle of blood typing is that an individual must receive one of the blood groups of each parent for each of the systems (loci). also, a stallion cannot be the sire of a particular foal unless the foal carries one of the stallion's blood groups at each of the blood group systems. For example, using the P system only, if a sire is of blood type a/B, he will pass on either the p^a gene (responsible for the A blood type) or the b^b gene (responsible for producing B blood type) to each of his progeny. A foal of blood type -/- (which is produced by the absence of the p^aand p^b alleles, in animals homozygous for alleles which don't produce detectable antigens) could not be the progeny of this stallion as it does not carry either of the two alleles of its sire. In such a case, the stallion in question is said to be excluded as a possible sire of this foal. In some cases, the dam's blood type may also provide useful information in determining whether or not a stallion could have sired a foal. For example, if a mare of transferrin type F₁ is the reported sire of this foal of DF₁ blood type, the reported sire is not actually the sire. Such a sire is said to be excluded as the sire of this foal. The actual sire of the foal must have provided a D allele and a horse of transferrin type F₁ cannot provide a D allele as had he possessed the allele, he would have expressed it.

Use of blood typing can resolve over 94% of the cases of disputed parentage. In other words, blood typing could successfully detect that a particular sire was not the sire of a foal. Often, breed associations require that blood tests be done to verify parentage in cases where the "chestnut or gray rules" are violated. This is to day, an association would not register a bay foal out of two chestnut parents without the foal's parents being verified through blood typing. The use of artificial insemination is allowed in some breeds and all foals resulting from it probably should be blood typed to verify the sire. embryos now can be transplanted from a donor mare to a recipient which then give birth to and raises the foal. Certainly in cases of embryo transplantation, the resulting foal, its reported sire and dam and the recipient mare should all be bloodtyped to determine that the foal isn't a progeny of the recipient mare. This seems to be the practice of all associations which allow embryo transfers.

Mating Systems

It is perhaps useful to begin a discussion of mating systems with a careful definition of the terms that will be used in the discussion. Mating systems include inbreeding, line breeding and crossbreeding. A working definition for inbreeding would be the mating together of animals whose pedigrees shown common ancestors within the first four generations (back to the great-great-grandparents). when the same stallion (Old Sorrel) which is the sire of the sire of a foal and also is the grandsire of the dam of the foal, he is a common ancestor.

Horses with no common ancestors for the first three or four generations back in the pedigree can be referred to as outcrosses. The term, outcross, is only used for the mating of unrelated animals of the same breed. Perhaps the most extreme outcrosses possible are by using imported stallions on mare with multiple generations of domestic breeding.

Inbreeding can be very intense, such as when a stallion in mated back to his daughter or quite mild such as is shown in the previous example (Figure 2). Line breeding is a type of inbreeding but the mated animals are generally not particularly closely related. The mated animals do, however, descent from the same ancestor.

One of the goals of line breeding is to try to preserve the genes and gene combinations of the animal which occurs several times in the pedigree. (Such as Sir Alex in the above pedigree). Line breeding is most frequently used as a mating system in species bred for exhibition purposes and which also, have relatively high rates of reproduction.

The genetic consequences of inbreeding is to reduce the frequency of loci, specific locations on chromosomes, which are heterozygous, that is, have genes of two different types. The result, increased homozygosity, means that animals resulting from line breeding programs could be somewhat more prepotent for their observed traits, particularly for structural traits. Prepotency means that the progeny produced are uniform. Unfortunately, this prepotency could yield progeny with consistent faults as well as predictable virtues. This is why it is so important to select (cull) those animals very carefully produced from an inbreeding program.

While the effects of inbreeding in horses have not been extensively studies, there have been numerous studies of its effects on other species beginning with studies of guinea pigs before 1920. Regardless of species, the general effects of inbreeding have been markedly consistent across species so there is no reason to suspect that comparable effects would not be observed in horses. Inbreeding has its greatest effects on fertility traits and traits related to viability such as the vigor of newborn foals. It has almost no effect on mature height. An inbred mare, on the average, will be more difficult to get in foal than noninbred (outcross) or crossbred mares. Such mares may not reach puberty as early as noninbred mares (this is not likely to be too critical to horse breeders as fillies are not usually bred for the first time until three years of age at the earliest and often much later). The problem of getting mares in foal, given the low reproductive rate of some breeds, could be of much greater concern. Completely sterile mare ( and stallions) could result from multiple generations of very close matings but would not be expected form the types of inbred matings likely to be made by horse breeders.

The mating of a half-brother to his half-sister, theoretically, results in an inbreeding coefficient (a measure of the reduction in heterozygosity) of .125. The progeny of such a mating would be expected to be 12.5% less heterozygous that average horses of that breed. This amount of inbreeding can be enough to effect fertility, newborn vigor and growth rate. The progeny of such matings or even closer matings such as sire-daughter matings, however, are not likely to show gross physical abnormalities (freaks, monsters) unless the same specific recessive genes controlling such conditions are present in both of the parents. Certainly, the probability of producing defective conditions controlled by recessive genes increases as a result of inbreeding. Such conditions will not be produced as a result of inbreeding, however, unless such recessive genes are present. since recessive genes responsible for gross defective conditions appear to be rare in horses, the chances that inbreeding will produce a dead foal or a foal with an observable abnormality at birth are not great. An exception to this might be the Arabian bred where the presence of the gene for combined immunodeficiency (a lethal condition when homozygous) may make inbreeding riskier.

Studies have shown that close inbreeding, is being avoided, in general, by breeders of Standardbred horses (MacCluer et al., 1983). In a detailed study of pedigrees dating back to 1800s, they showed that while inbreeding was not increasing in this breed, nearly all current Standardbred horses are inbred to some degree. The average inbreeding coefficient of horses born in the early 1980s was nearly .09. For comparative purposes, the progeny resulting from the mating of half-brother to half-sister have an inbreeding coefficient of .125 and those from the mating of cousins is .0625.

One of the reasons the inbreeding coefficients of the Standardbreds are so high is because the effects of past inbreeding, when smaller numbers of stallions may have been available and used, still impact the breed since the herdbook is closed. As a result of having a closed herdbook, no new, totally unrelated animals can be crossed into the breed to relieve the possible negative effects of past inbreeding. The effects of a small initial number of stallions which founded a breed, or a single stallion which had a major impact on the formation of the breed, such as the Justin Morgan stallion which founded the Morgan breed, can have a permanent impact on the inbreeding of the resulting breed. In the case of the Morgan Horse, however, so many different type of mares were used in the breed's development that inbreeding is not expected to be particularly high.

It should also be emphasized that there can be desirable effects of inbreeding. In many species bred for their physical traits (type traits), such a exhibition poultry, rabbits, dogs, cats, etc., inbreeding or line breeding are very commonly practiced. In fact, more inbred matings likely are made than outcross matings in most of the above mentioned species. The reason for such breeding practices is that the judicious use of inbreeding or line breeding can produce lines of animals which are very uniform in physical appearance and whose offspring, generation after generation, are predictably of this same type. The reason for this uniformity is the increased homozygosity which results from the inbreeding process. In order to develop such uniform, predictable lines, however, the breeders of these species have had to cull heavily for a number of generations in conjunction with the use of inbreeding. It is the inability of breeders of horses to cull as severely (due to much smaller number of progeny produced) that prevents the similar use of inbreeding in horses.

Finally, it should be mentioned that inbreeding usually does not produce the "monsters" or "crazy' animals in spite of the fact that this seems to be commonly believed by the general public. Inbreeding, as mentioned previously, simply increases homozygosity. Only when horses are inbred which carry recessive genes which, when homozygous, produce defects in animals will the practice of inbreeding result in increases in the frequency of foals being born with observable defects at birth. Since such defective genes appear to be rare, the probability that an occasional inbred mating, either accidental or planned, will produce a "monster" is quite remote. A much more frequent occurrence will be a slight reduction in vigor and fertility which cannot be measured except across a number of animals.

The Use of Crossbreeding in Horses

Crossbreeding is not frequently utilized in horses so an extensive discussion of crossbreeding in this text is not justified. The American Quarter Horse Registry registers crossbreds of Quarter Horses and Thoroughbreds (provided certain requirements are met) and stallions of the Dutch Warmblood breeds have been crossed with Thoroughbred mares to produce "hunter-jumper" prospects. The use of crossbreeding of the Morgan and Arabian breeds to produce pleasure horses, the Percheron and Thoroughbred breeds to produce hunter-jumpers, and the Cleveland Bay and Thoroughbred breeds to produce performance horses was discussed by Follmer (1980). The goal is to produce a combination of temperament, structure and endurance that cannot be found within any of the available breeds. The genetic effects of crossbreeding are simply the opposite of those of inbreeding. Whereas inbreeding reduces heterozygosity and increases homozygosity, crossbreeding increases heterozygosity and reduces homozygosity. The degree to which crossbreeding increases heterozygosity depends on the genetic differences between the breeds being crossed. The Quarter Horse, being derived initially from Thoroughbred and other breeding, is quite genetically similar to the Thoroughbred. This fact was documented by Bowling and Clark (1985) who compared the blood group and protein polymorphism gene frequencies of breeds of horses in the U.S. As a result of this similarity of genetic background, crosses of Quarter Horses and Thoroughbreds are not likely to show as much of the effects of increased heterozygosity as would crosses of more distantly related breeds.

The effects of increased heterozygosity as a result of crossbreeding has not been studies in horses. Results of crossbreeding studies in almost all other species of domesticated mammals, however, have consistently shown improvements in traits relating to reproduction and newborn vigor. Increases are usually also seen in the growth rate of young animals although mature size is generally not greatly affected. Crossbred animals mature faster as a result of faster growth early in life whereas purebred animals (especially inbred ones) grow for a longer period of time (are late maturing) and as a result reach comparable weights. Similar results would be expected in crossbred horses.

Selection of Horses

Selection, in its broadest sense, includes the entire process of deciding which stallions will be kept intact to use for breeding animals and which mares will be bred. Selection in horses is usually practiced for a number of traits simultaneously, including conformation, speed (in racing breeds), size, temperament, etc. Variation naturally occurs in each of these traits and the process of selection involves evaluation of animals for each of the traits of interest and then making decisions regarding the animals based on their composite evaluation. Selection decisions can be made both subjectively such as is done during the evaluation of conformation and temperament or objectively for traits such as size or speed which can be measured with a "Yard stick" or stopwatch. For traits which can be measured objectively, there is a wealth of information available regarding the selection process from other species of domestic animals. The understanding of the selection process to improve objectively measured traits gained from other species certainly can be applied directly to the breeding of horses. To this point, however, such selection principles have not been applied by horse breeders (for the most part) for a number of reasons, some of which will be discussed.

Procedures have been developed by the diary industry to select in a scientific manner for a number of conformation traits of dairy cows. Daughters of many bulls are evaluated for their physical traits, feet and leg structure, udder qualities, body capacity and others using a linear scale. By a linear scale, it is meant that straight legs (postlegged) may be given a score of 10, extremely crooked (sickle-backed) a score of 1, with properly angulated animals being given a score of 5. Animals with slightly crooked legs would be scored 3 and ones with somewhat straighter legs then would be considered desirable (not enough angulation) might be scored a 7 or an 8. After being analyzed, these data provide accurate numerical data on the strengths and weaknesses of the daughters of the bulls. Therefore, breeders with cows that are undesirable in a particular area, such as weak pasterns, can select a bull to breed to that cow whose daughters are known to have strong pasterns. As a result, the likelihood of improving the pastern strength of the resulting progeny are improved. Breeders of horses, on the other hand, have much less information, and no truly unbiased information upon which to make decisions regarding choices of sires. They must depend upon the information provided by stallion owners regarding the sires strengths and weaknesses. Information regarding weaknesses is likely to be less readily divulged than information on strengths. even the sire owner who is completely straightforward with mare owners is often limited by his inability to evaluate the progeny of his stallion. This is due to the fact that his stallion may not have produced many progeny that the owner has seen when old enough to properly evaluate. word of mouth information from other persons familiar with the strengths and weaknesses of the progeny of a particular sire has similar weaknesses. Such persons may base their opinions on one or at least a small number of progeny and, of course, can be biased wither in favor or against a particular stallion due to many causes. Such problems can be overcome if a system similar to that of the dairy industry was to be adopted. while it would be difficult to accumulate the data necessary to incorporate such a system for stallions and would require the strong support of breed registry organizations, it certainly could be accomplished.

The amount of selection progress (change in a trait as a result of selection) is determined by three factors. These factors include what are referred to as the accuracy of selection, the intensity of selection and the genetic variability of the trait. Each of these factors will be explained and the means by which they could be manipulated to increase the amount of genetic progress will be discussed.

The accuracy of selection is a measure of the correlation (relationship or association) between the horse's genotype and the phenotypic measures (weight, height, etc.) used to estimate it. It indicates the reliability of the information used to predict the animal's genotype. We can never know the true genetic potential of an animal, but through measures of its own performance or that of relatives, particularly progeny, we can estimate it with varying degrees of accuracy. The degree of accuracy of evaluation based on the performance of the horse for the trait is dependent on the heritability of the trait being considered. The accuracy of selection based on relative's performance (sire, dam, half sibs, full sibs, progeny) depends on the degree of relationship (full vs. half sib, for example) and number of sibs or progeny. Accuracy increases as the heritability of the trait increases and as the number of progeny evaluated increases.

The heritability of a trait can be defined in broadest terms as the degree to which a trait is genetically controlled. The heritability determines the proportion of the variation in the phenotypic measures (heights or weights for example) that can be accounted for by heretic variation as opposed to that variation accounted for by the environment. As it is a proportion, heritability values fall between 0, indicating not genetic control over the phenotypic performance, to 1, indicating complete genetic control over the phenotype. In practice, heritability values generally fall between .05 to .15 for most reproductive traits (in other domestic animals species) to a high of about .6 for structural traits such as mature height. The portion of the phenotypic variation that is not genetically controlled is due to environmental factors. Environmental factors include nutrition, disease, climatic conditions, training procedures, as well as everything that influences a trait that is not genetic. The heritabilities of some traits of horses are shown in Table 7. Rather wide ranges are shown for most traits. It is not uncommon for heritability estimates calculated for the same trait in other domestic species to vary from one study to another using a different breed or herd of the same breed. In general, wide ranges are shown as many factors can influence the heritability of a trait under given circumstances and methods of its estimation. The values shown are based on estimates from the literature an in some cases from extrapolation from other species where similar traits have been studied more intensely. Few studies have been carried out to determine heritability in horses. Certainly traits related to reproductive efficiency in horses (such as services per conception) are going to be of rather low heritability since they have been found consistently to be low in all other species that have been investigated. Other traits, especially those related to height and weight, are much more highly heritable. Mature height is likely to have a heritability of at least .50 to .70. Traits such as trainability and speed certainly are going to be intermediate between the two extremes of reproduction and height and will likely be in the range of .20 to .40.

Table 7. Heritabilities^a of Traits of Horses

Trait Heritability

Reproductive efficiency

Speed

Wither height

Conformation traits

Trainability

Pulling Power
.05 - .20

.20 - .40

.50 - .70

.20 - .40

.15 - .40

.10 - .30

^aHeritability ranges are based on literature estimates or by extrapolation from heritabilities from other species.

An obvious question to ask then is how do the heritability values affect whether or how one practices selection. Since the amount of progress we can expect due to selection is directly related to the heritability of the trait according to the equation below:

Expected Genetic Change = Heritability × Selection Differential

It can readily be seen that if the heritability of a trait is low, there will be little progress regardless of the selection differential.

The selection differential is the deviation of the mean performance of the stallions and the mares kept as breeding animals over that of all contemporary animals. Actually, the selection differential is almost never equal in both sexes, being much greater in stallions than mares since one stallion can be bred to 40 or more mares per year (even more, of course, with artificial insemination). As a result, the average of the selected stallions will be further away from the mean of all stallions than the comparable average of selected mares from the average of all mares. In order to calculate an overall selection differential, one simply averages the selection differentials for the stallions and the mares selected since each contributes equally to the progeny from a genetic standpoint.

As an example, let us consider selection for height. If the average for a group of stallions is 15 hands and stallions are selected which average 16 hands, then the selection differential for the sires is 1 hand. The selection differential of the dams must also be considered and would be .5 hands if the selected mares average 15.2 hands (15 hands, 2 inches) and all mares average 15 hands. The overall selection differential is the average of the selection differentials for the sires and the dames. In this example, it is .75 hands, ((1 + .5) ÷ 2). If we assume a heritability for height of .6, we would expect the genetic progress to this selection for height to be .6 × (.75) or .45 hands. In other words, we would expect the average height of the progeny of these horses selected on the basis of height to be about 2 inches (.45 hands) taller than the average horses from which the parents were selected (15 hands). Thus, the average height of the progeny of the selected stallions and mares would be 15.45, (15, the original height, plus .45), the genetic progress.

The selection differential can be increased through the use of artificial insemination as this technique allows a stallion to sire more progeny, thus, lowering the number of sires needed. When fewer sires are needed, the breeder selects sires that on the average, surpass the mean of the population by a greater quantity which increases the selection differential and, therefore, genetic progress if other factors remain unchanged. Similarly, it were possible to register more than one progeny per year from mares through use of embryo transfers, a modest increase in the selection differential for mares could be utilized.

For traits with relatively low heritability values, such as some measure of trainability perhaps, the accuracy of selection can be improved somewhat by measuring the trait more than once and then evaluating the horses based on their average performance. The accuracy of selection based on the average of repeated measures is given by the formula:

where

h = the square root of the heritability of the trait,

n = the number of times the trait was measured and

R = the repeatability of the trait being measured.

A trait's repeatability is the correlation between the repeated records. This is, how reliable is one record as an indicator of future records.

The accuracy of selection when the trait is measured just one is the square root of h²or h. If the h² of trainability is .25 then the accuracy of selection is .5. If we repeat the evaluation of trainability five times and the correlation between these repeated measurements is .5, and then, rank horses based on the average of the five measurements, then the accuracy of selection, based on this average, is increased to .645. As a result we can select animals 29% more accurately by use of the average rather than with just one measurement.

Accuracy can also be increased through progeny testing. This is evaluation of a horses' genetic merit based on the performance its progeny. This is done to a degree by the Thoroughbred industry in its "sire of stakes winners" reporting. While this type of evaluation is useful, it lacks the accuracy of the "sire summary" information available on beef and dairy bulls for several reasons. First, the number of progeny on which information is available is rather low; secondly, there is no adjustment of progeny results based on the quality of the mares the stallion is bred to; and thirdly, the progeny records are not adjusted for environmental factors such as age of the progeny (January versus May foals), sex of the foal, track condition, gate position and other factors which could influence whether or not a stallion's progeny win races. A more useful measure than percent winners or, in addition to percent winners, would be some actual measurement of speed on all the progeny of a horse in comparison to the speeds of the progeny of other stallions trained and timed under similar conditions. The progeny of evaluation of a stallion's genotype for speed can be calculated using an h² of speed of .4 would be :

where

n = the number of progeny with speed records

h² = heritability of speed = .4

If we use as an example n = 20, perhaps the number of progeny from a single foal crop, the accuracy of selection increases to .83 from .63 (that possibly from evaluation of the sire based on his own speed, ), an increase of over 31%. With 60 progeny the accuracy of selection rises to .93 and thus, we can very accurately evaluate the genetic potential for speed of a stallion.

The major drawbacks of progeny testing are that only a fraction of all stallions born can be progeny tested due to economic factors and the simple lack of enough mares to be bred to each young stallion and the length of time that it takes to evaluate each stallion. A stallion which is first mated as a four-year-old will have his first progeny born when he is five years of age. By the time three foal crops have reached two years of age and been evaluated at two years of age, the stallion is nearly ten years old. Thus, we cannot determine which stallions are the best until they are rather old. If the superior older stallions are used in lieu of unproven younger stallions, the generation interval, essentially the average age of the breeding animals, is increased. The problem with increasing the generation interval is that it can slow the rate of genetic progress. In order for progeny testing to be most useful it must increase the accuracy of selection more than it increases the generation interval. This is most likely to occur for more lowly heritable traits. Traits related to speed probably fall in this category.

The question as to whether or not current methods of selecting racehorses for increased speed is being effective is an interesting one. If we assume that track conditions, nutrition, disease control, trailing methods and riding techniques have at least remained constant over the past 50 years, and genetic improvement for speed has occurred, we would expect the record times for a given distance or at least the average time to cover a given distance to steadily decline. The former, at least, does not appear to be happening. This question was recently examined thoroughly in England by W.G. Hill (Nature 332:678) and B. Gaffney and E.P. Cunningham (Nature 332:772-724). Hill observed that the winning times of thoroughbreds in English classic horse races had not fallen substantially in the last 50 years, in spite of considerable attention being paid to performance and pedigrees in the selection process. Gaffney and Cunningham found estimates of racing performance to range from .39 to .76. They also observed that the selection differentials which had been practiced over the years were quite intense, essentially the 6% fastest stallions and approximately the top half (52%) of the mares were used to produce progeny. Generation intervals, however, are quite long, about 10 years, but, in spite of this, some increases in speed would have been expected. Thus, the question arises as to why times have not fallen if there has been selection of the best sires and dams and substantial heritabilities for speed.

Comparing the structure of the Thoroughbred industry to that of the dairy industry may be useful in determining some of the causes of lack of progress. The rate of genetic progress in milk production is about 1% per year. This is in spite of the fact that milk yield is lowly heritable (~.20) and can be measured in only one sex. Speed, on the other hand, appears to be somewhat more highly heritable and can be evaluated in both sexes. So, what are the differences. First of all, many Thoroughbred horses may not be evaluated for their racing ability as they never get to the track for a variety of reasons. Secondly, the information available to horse breeders to use in making decisions about sires is quite limited and likely is much less accurate than that available to dairymen regarding diary bulls. The reasons for this, some of which have been discussed earlier, include small numbers of progeny per sire, lack of adjustment of progeny data for nongenetic factors, and a lack of adjustment for the quality of the mares to which each of the stallions are bred.

Even when a stallion has been identified as a proven sire of winners and likely genetically superior to other stallions, the lack of availability of artificial insemination restricts severely the number of progeny the stallion can produce annually. As a result, mares are bred to younger or inferior stallions and thus, genetic progress can be reduced. Also, long generation intervals further reduce possibilities for genetic change.

While it will not be possible to have much effect on the generation interval, changes in the evaluation of stallions could be accomplished. This alone, especially if accompanied by use of artificial insemination to allow greater usage of superior sires, could have the effect of increasing genetic progress for speed.

An additional explanation for a lack of an increase in speed is that horses' speed has reached a physiological limit, that horses cannot go faster without injuring themselves. If this were true, we might expect more injuries in the progeny of the best sires which would, in theory, be the fastest horses. Data are not readily available, however, to substantiate or refute this suggestion.

Mating Decisions

A challenge which faces the breeder of horses each year is how to choose the stallion to breed to each mare. A practice commonly used is referred to as "crossfaulting." Crossfaulting can be defined as the mating of mares with defects in certain areas to stallions known to be strong in that area, at least by visual appraisal. For example, if your mare is too small, you would probably want to breed the mare to a large stallion. A mare which is too frail, with bone which is too light, should be bred to a strong heavy-boned stallion. Other examples might be back length were a mare considered to be too long in back would be bred to a stallion which is somewhat short-backed. While evaluations of the progeny of such stallions to ensure that they resembled the characteristics of their sire would be superior to use for evaluation of the stallion himself, most physical traits are sufficiently highly heritable to make individual evaluation of the traits of stallions themselves useful.

A common mistake made by breeders of all types of animals is to place too much emphasis on ancestors which are too far back in the pedigree to have a significant impact upon the animals in question. As an example, one should not be particularly impressed with the fact that a young horse has Northern Dancer as a great-grandsire. On the average an animal will have one-eight of its genes from each of its eight great-grandparents. This is on the average, however, the individual may not have any influence or very little influence from any particular great-grandparent. While a grandparent may have a major impact on a particular individual, it is usually best not to place much emphasis on ancestors other than parents. Even the mating to a son of a stallion known to be particularly prepotent for a given trait is risky if the son has not been shown to also excel in the trait.

Another pitfall to avoid is the tendency to try to select for too many traits at the same time. Considerable progress can usually be make if selection is applied only to one trait. Each additional trait that is considered reduces the amount of selection pressure that can be placed on each individual trait. If four traits are selected for, the selection intensity possible for each individual trait is reduced to one-half of what it would have been had it been the only trait selected. No traits, however, that significantly effect the value of utility of your horses should be ignored in the selection process.

To summarize, it is recommended that horse breeders use as much information as they can obtain in evaluation of stallions to use for breeding. The stallions won performance or phenotype, which includes his racing ability, physical traits or other measures of his utility should be considered. Even more important, however, is the performance of his progeny; particularly in comparison to the progeny of other stallions which had been bred to similar quality mares. Increasing the information regarding a stallion's siring ability that is available allows the mare owner to make more appropriate matings and as a result, superior progeny should be produced.

Literature Cited

Bowling, A.T. and R.S. Clark. 1985. Blood group and protein polymorphism gene frequencies for seven breeds of horses in the United States. Anim. Blood Grps. Biochem. Gen.16:93-108.

Follmer, D. 1980. A nation of crossbreds: the melting pot still bubbles. Equus 36:24-28.

Gaffney, B. and E.P. Cunningham. 1988. Estimations of genetic trend in racing performance of Thoroughbred horses. Nature 332:722-724.

Hill, W.G. 1988. Why aren't horses faster? Nature 332:678.

MacCluer, J.W., A.J. Boyce, B. Dyke, L.R. Weitkamp, D.W. Pfenning and C.J. Parson. 1983. Inbreeding and pedigree structure in standardbred horses. J. Hered. 74:394-399.

Sponenberg, D.P. and B.V. Beaver. 1983. Horse Color. Texas A&M University Press.

Sponenberg, D.P., H.T. Harper and A.L. Harper. 1984. Direct evidence for linkage of roan and extension loci in Belgian horses. J. Hered. 75:413-414.

Sponenberg, D.P. and E. Kilby. 1987. A few colors that can kill (and a lot of look-alikes that don't). Equus 112:38-42, 82.

Trommershausen-Smith, A., Y. Suzuki and C. Stromont. 1976. Use of blood typing to confirm principles of coat color genetics in horses. J. Hered. 67:6-10.

Trommershausen-Smith, A. 1980. Aspects of genetics and disease in the horse. J. Anim. Sci. 51:1087-1095.

Woolf, C.M. 1989. Multifactorial inheritance of white facial markings in the Arabian horse. J. Hered. 80:173-178.

Suggested Further Reading

Buttram, S.T., R.L. Willham and D.E. Wilson. 1988. Genetics of racing performance in the American Quarter Horse: I. Description of the data. J. Anim. Sci. 66:2791-2799.

Buttram, S.T., R.L. Willham and D.E. Wilson. 1988. Genetics of racing performance in the American Quarter Horse: II. Adjustment factors and contemporary groups. J. Anim. Sci. 66:2800-2807.

Buttram, S.T., D.E. Wilson and R.L. Willham. 1988. Genetics of racing performance in the American Quarter Horse: III. Estimation of variance components. J. Anim. Sci.66:2808-2816.

Cothran, E.G., J.W. MacCluer, L.R. Weitkamp, D.W. Pfenning and A.J. Boyce. 1984. Inbreeding and reproductive performance in Standardbred horses. J. Hered. 75:220-224.

Ojala, M.J. and L.D. Van Vleik. 1981. Measures of racetrack performance with regard to breeding evaluation of trotters. J. Anim. Sci. 53:611-619.

Ojala, M.J., L.D. Van Vleik and R.L. Quaas. 1987. Factors influencing best annual racing time in Finnish horses. J. Anim. Sci. 64:109-116.

Sponenberg, D.P. 1982. The inheritance of leopard spotting in the Noriker horse. J. Hered. 73:375-359.

Stromont, C. and Y. Suzuki. 1964. Genetic systems of blood groups in horses. Genetics50:915-929.

Tolley, E.A., D.R. Notter and T.J. Marlowe. 1985. A review of the inheritance of racing performance in horses. Anim. Breed. Abstr. 53:163-185.

Wilson, D.E., R.L. Willham, S.T. Buttram, J.A. Hoekstra, and G.R. Luecke. 1988. Genetics of racing performance in the American Quarter Horse: IV. Evaluation using a reduced animal model with repeated records. J. Anim. Sci. 66:2817-2825.

Woolf, C.M. and J.R. Swafford. 1988. Evidence for eumelanin and phaeomelanin producing genotypes in the Arabian horse. J. Hered. 79:100-106

Mutant Name/Description	Symbol	Pattern Relative to Wild Type^a
Black (will fade in intense sunlight)	a	Recessive
Sorrel (no black hairs in coat)	e	Recessive (but epistatic to the A locus)
Cremello (dilution of red or reddish colored hairs, homozygotes are nearly white)	c^cr	Incompletely dominant
Dilution of red, reddish or black hairs to dun, red dun or grulla	D	Dominant
Pangaré (dilution of sorrel to blond sorrel, black to seal brown)	P	Dominant

Matings/Genotypes	Expected Progeny Phenotype/Genotypes	Matings/Genotypes	Expected Progeny Phenotypes/Genotypes
Sorrel × Sorrel (ee-- × ee--)	100% sorrel (ee--)	Bay × Bay (E⁺eA⁺A⁺ × E⁺eA⁺A⁺)	75% Bay (E⁺E⁺A⁺A⁺ or E⁺eA⁺A⁺) 25% Sorrel (eeA⁺A⁺)
Bay × Bay (E⁺E⁺A⁺A⁺ × E⁺eA⁺A⁺)	100% Bay (E⁺E⁺A⁺A⁺ or E⁺eA⁺A⁺)	Bay × Sorrel (E⁺E⁺A⁺A⁺ × eeA⁺A⁺)	100% Bay (E⁺eA⁺A⁺)
Bay × Sorrel (E⁺eA⁺A⁺ × eeA⁺A⁺)	50% Bay (E⁺eA⁺A⁺) 50% Sorrel (eeA⁺A⁺)	Black × Black (aaE⁺E⁺ × aaE⁺E⁺)	100% Black (aaE⁺E⁺)
Black × Black (aaE⁺e × aaE⁺e)	75% Black (aaE⁺E⁺ or aaE⁺e) 25% Sorrel (aaee)	Sorrel × Bay (aaee × A⁺aE⁺e)	25% Bay (A⁺aE⁺e) 25% Black (aaE⁺e) 50% Sorrel (A⁺aee or aaee)
Sorrel × Bay (aaee × A⁺aE⁺E⁺)	50% Black (aaE⁺e) 50% Bay (A⁺eE⁺e	Sorrel × Bay (aaee × A⁺A⁺E⁺E⁺)	100% Bay (A⁺aE⁺e)
Black × Sorrel (aaE⁺e × A⁺A⁺ee)	50% Bay (A⁺aE⁺e) 50% Sorrel (A⁺aee)	Black × Sorrel (aaE⁺E⁺ × A⁺A⁺ee)	100% Bay (E⁺eA⁺A⁺)
Black × Sorrel (aaE⁺e × A⁺aee)	25% Bay (A⁺aE⁺e) 25% Black (aaE⁺e) 50% Sorrel (A⁺aee or aaee)	Bay × Palomino (A⁺A⁺E⁺C⁺C⁺ × A⁺A⁺eeC⁺c^cr)	50% Bay (A⁺A⁺E⁺eC⁺C⁺) 50% Buckskin (A⁺A⁺E⁺eC⁺c^cr)
Bay × Palomino (A⁺A⁺E⁺eC⁺C⁺ × A⁺A⁺eeC⁺c^cr)	25% Bay (A⁺A⁺E⁺eC⁺C⁺) 25% Buckskin (A⁺A⁺E⁺eC⁺c^cr) 25% Palomino (A⁺A⁺eeC⁺c^cr) 25% Sorrel (A⁺A⁺eeC⁺C⁺)	Sorrel × Palomino (A⁺A⁺eeC⁺C⁺ × A⁺A⁺eeC⁺c^cr)	50% Palomino (A⁺A⁺eeC⁺c^cr) 50% Sorrel (A⁺A⁺eeC⁺C⁺)
Black × Palomino (aaE⁺E⁺C⁺C × A⁺A⁺eeC⁺c^cr)	50% Bay (A⁺aE⁺eC⁺C⁺) 50% Buckskin (A⁺aE⁺eC⁺c^cr)	Black × Palomino (aaE⁺E⁺C⁺C⁺ × A⁺aeeC⁺c^cr)	25% Bay (A⁺aE⁺eC⁺C⁺) 25% Buckskin(A⁺aE⁺eC⁺c^cr) 50% Black (aaE⁺eC⁺C⁺ or aaE⁺eC⁺c^cr)
Black × Palomino (aaE⁺eC⁺C⁺ × A⁺aeeC⁺c^cr)	12.5% Bay (A⁺aE⁺eC⁺C⁺) 12.5% Buckskin (A⁺aE⁺eC⁺c^cr) 25% Black (aaEeC⁺C⁺ or aaEeC⁺c^cr) 25% Palomino (A⁺aeeC⁺c^cr or aaeeC⁺c^cr) 25% Sorrel (A⁺aeeC⁺C⁺ or aaeeC⁺C⁺)	Palomino × Palomino (A⁺A⁺eeC⁺c^cr × A⁺A⁺eeC⁺c^cr)	25% Sorrel (A⁺A⁺eeC⁺C⁺) 50% Palomino (A⁺A⁺eeC⁺c^cr) 25% Cremello (A⁺A⁺eec^crc^cr)
Palomino × Buckskin (A⁺A⁺eeC⁺c^cr × A⁺A⁺E⁺eC⁺c^cr)	12.5% Bay (A⁺A⁺E⁺eC⁺C⁺) 12.5% Sorrel (A⁺A⁺eeC⁺C⁺) 12.5% Perlino (A⁺A⁺E⁺ec^crc^cr) 12.5% Cremello (A⁺A⁺eec^crc^cr) 25% Buckskin (A⁺A⁺E⁺eC⁺c^cr) 25% Palomino (A⁺A⁺eeC⁺c^cr)	Dun × Bay (A⁺A⁺E⁺E⁺Dd⁺ × A⁺A⁺E⁺E⁺d⁺d⁺)	50% Dun (A⁺A⁺E⁺E⁺Dd⁺) 50% Bay (A⁺A⁺E⁺E⁺d⁺d⁺)

Locus
Albumin	Transferrin	Postalbumin	Prealbumin	Esterase
A	D	F	2	F
B	F₁	S	3	G
	F₂		4	H
	H₁		5	I
	H₂		6
	O		7
	R