Horse GeneticsEquine Science & Technology

Horse GeneticsNature ordained that genetics be applied to horse breeding

long before there were geneticists. Prior to the domestication of horses, there was natural selection for speed and stamina. One of the most important defense mechanisms of the horse was to outrun its enemies.

Natural selection was probably very effective in improving speed and endurance, since the slower horses were eliminated by predators.

Horse Genetics Many horse breeders have been practicing genetics as they

concern themselves with the are of breeding. Their guiding concept of heredity is that “like begets like”.

Breeders in the 18th century made a tremendous contribution pointing the way toward horse improvement before Mendel’s laws became known to the world.

As knowledge of genetics developed, there evolved an understanding of the science that underlies the art of horse breeding.

Horse Genetics Genetics The bodies of animals are made up of millions or even

billions of microscopic cells. Each cell contains a nucleus in which there are a number of

pairs of bundles called chromosomes. In turn, the chromosomes carry pairs of minute particles

called genes, which are the basic hereditary material. The nucleus of each cell of a horse contains

32 pairs of chromosomes, or a total of 64.

Horse GeneticsGenetics There are thousands of pairs of genes on each

chromosome. Genes determine all the hereditary characteristics of

animals from the body type to the color of the hair. The mule has an uneven number of chromosomes and

cannot reproduce. The genotype of an animal refers to the specific genes it

possesses on its chromosomes. Genome describes the complete set of “instructions” for

making an organism.

Horse GeneticsGenetics Dominance gene expression has a full allele effect. An

example would be when a gray horse produces a gray horse.

Recessive character appears only when both members of a pair of alleles are alike.

The job of transmitting qualities from one generation to the next is performed by the gene cells- a sperm from the male and an ovum, or egg from the female.

Horse GeneticsGenetics There are alternate forms of each gene. The alternate gene

forms are referred to as alleles. If the alleles are the same, the condition is referred to as homozygous. If the alleles are different, the condition is called heterozygous.

Selection and crossbreeding are the tools through which the horse breeder may obtain stallions and mares whose chromosomes and genes contain similar hereditary determiners-animals that are genetically more homozygous.

Horse GeneticsGenetics Gene changes are known as mutations. A mutation may

be defined as a sudden variation that is later passed on through inheritance and that results from changes in a gene or genes.

Horse GeneticsSimple Gene Inheritance (Qualitative Traits) In the simplest type of inheritance only one pair of genes is

involved. Thus, a pair of genes may be responsible for the color of body hair in horses.

The idea that certain basic colors may have a rather simple explanation of inheritance should not alter the fact that other genes may play an important role through their influence on basic schemes.

The possible gene combinations are governed by the laws of chance.

Horse GeneticsSimple Gene Inheritance (Qualitative Traits) Other possible examples of simple gene inheritance in

horses might include eye color and the set of the ears on the head.

Horse GeneticsDominant and Recessive Alleles A dominant allele may cover up a recessive allele.

Therefore, a horse’s breeding performance cannot be recognized by its phenotype (how it looks).

Recessive genes can be passed on from generation to generation, appearing only when two animals, both of which carry the same recessive allele, happen to mate.

Horse GeneticsDominant and Recessive Alleles Reputable breeders have an obligation, not only to

themselves, but to their customers. Purebred animals must be purged of undesirable genes by eliminating those stallions and mares that are known to have transmitted the undesirable recessive gene.

Horse GeneticsIncomplete Dominance In some cases, dominance is neither complete nor absent,

but incomplete or partial and expresses in a variety of ways.

Perhaps the best known case of this type in horses is the Palomino color. Genetic studies of the Palomino indicate that the color is probably unfixable- that it cannot be made true breeding, no matter how long or how persistent the effort.

Horse GeneticsMultiple Gene Inheritance (Quantitative Traits) Important characters such as speed are due to many genes;

thus, they are called multiple-gene characters. Quantitative inheritance refers to the degree to which a

characteristic is inherited. For example, all Thoroughbred horses can run and all

inherit some ability to run, but it is the degree to which they inherit the ability that is important.

In quantitative inheritance, the extremes (either good or bad) tend to swing back to the average.

Horse GeneticsHorse Color Recent advances in genetics have allowed us to

understand better how coat color is inherited, thus making it possible to plan matings that will produce foals of a certain color.

Horse GeneticsGene A Genetic control of black color restricted to the points of a

horse is through gene A, or the A allele. The A allele is dominant. A uniformly black horse has the recessive genotype aa.

Horse GeneticsGene E Gene E, or the E allele, controls the expression of black

hair on the body. The recessive form ee, results in uniformly red-colored

horses. The E gene is related to (and can mask) the A gene.

Horse GeneticsGene G Gray coloring is due to the dominant gene G. Any horses with the G allele will be gray; non-gray horses

will have the genotype gg. Gray horses can be born any color.

Horse GeneticsGene W Solid white coloring is controlled by gene W. The dominant homozygous (WW) condition is lethal; thus,

all white horses have the genotype. All non-white horses have the genotype ww.

Horse GeneticsGene C and D The C and D alleles are dilution alleles. Gene C affects

only red pigment and causes the pigment to lighten. The C gene does not affect black pigment. Gene D differs from gene C in that it can dilute both black

and red pigment. With the D allele, black is diluted to a mouse or slate grey,

and red is diluted to a yellow-tan. Horses with the D allele also show primitive marks such as

the dorsal stripe, shoulder stripes, and leg barring.

Horse GeneticsGene TO The spotting of paint and pinto horses is controlled by the

tobiano gene TO. The tobiano color pattern is dominant; thus, overos must

have the genotype toto. Homozygous TOTO horses are rare and are sought after by

horse breeders for their ability to always produce spotted foals.

Genetic tests are available to determine whether a horse is homozygous.

Horse GeneticsHeritability of Performance Traits Relatively little scientific work has been done on the

heritability of performance of horses- on the genetics of working ability, racing ability, cutting ability, etc.

The horse is the last farm animal to which the science of genetics has been added to the art of breeding.

Differences in the performance ability (working, racing, jumping) are due to two major forces-heredity and environment.

Horse GeneticsHeritability of Performance Traits The important environmental factors in determining the

overall performance of horses are nutrition (both prenatal and postnatal), health care, quality of training, ability of the handler, and injuries.

An important genetic principle is that traits as such are not inherited. The ability to respond to a given set of environmental conditions in order to produce a trait with a measurable effect is inherited.

Reliable estimates on the heritability of performance traits in horses are limited in comparison with those for other species.

Horse GeneticsThe following conditions would tend to indicate a

hereditary defect in horses: If the defect had previously been reported as hereditary in

the same breed. If it occurred more frequently within certain families or

when there had been inbreeding. If it occurred in more than one season and when different

rations had been fed.

Horse GeneticsThe following conditions might be accepted as indications that

the abnormality was due to a nutritional deficiency: If it had previously been reliably reported to be due to a

nutritional deficiency. If it appeared to be restricted to a certain area. If it occurred when the ration of the dam was known to be

deficient. If it disappeared when an improved ration was fed.