INBREEDING's purpose is to fix certain traits or the influence of
certain ancestors upon the progeny. This procedure varies in degree from
intense closebreeding to mild linebreeding. Although inbreeding can be
detrimental to fertility, vigor, and athletic ability within the offspring, it
can also result in true-breeding strains of horses (that consistently pass
important traits to their offspring). Because most breeds were formed by a
process of inbreeding, the breeding of purebred horses is, my definition, a
form of inbreeding. Some breeds are more inbred than others. (Degree of
inbreeding depends on the number of common ancestors, how far back in the
pedigree they appear, and how often each common ancestor occurs.)
From a genetic viewpoint, inbreeding results in an increase of the number
of homozygous gene pairs in the offspring. Homozygous refers to a condition
where two paired chromosomes have the same allelle (gene type) at a
corresponding point. Because two close relatives tend to have more of the same
alleles (by virtue of inheritance) than two unrelated individuals, their
mating provides a greater chance for identical alleles to be paired within
their offspring. This increase in homozygosity is directly related to the
appearance of both desirable and detrimental characteristics that were not
necessarily apparent in the sire and dam.
When horses are inbred haphazardly, without culling of inferior stock, many
undesirable traits may become predominant in their offspring. For example, the
inbred horse's ability to resist disease and his overall performance capacity
are often depressed. The growth rate of the inbred foal, and the average
mature size within the inbred herd, frequently decreases. Nonselective
inbreeding is directly related to a depressed fertility rate, an increase in
abortion and stillbirth. Some basic principles of genetics show why these
traits are directly related to inbreeding.
When two unrelated horses are mated, the chances of unidentical alleles
combining within the resulting embryo are high. On the other hand, mating
close relatives increases the pairing of identical alleles (increases
homozygosity). The effect of increased homozygosity is a decrease in the
number of heterozygous gene pairs and, subsequently, a decline in heterosis
(i.e., loss of vigor and fertility). Although the reason for this allelic
interaction is not clear, geneticists believe that its presence contributes to
the overall quality of an individual. Therefore, as homozygosity increases
within the inbred herd, physical quality controlled by over dominant alleles
Many undesirable genes affecting the horse's overall vigor and fertility
are recessive. Fortunately, they have no influence in the heterozygous state,
since the effect of the recessive allele is completely hidden by the effect of
the corresponding dominant allele. Because of the overall effect of inbreeding
is an increase in homozygosity, it increases the number of homozygous
recessives. Hence, the effects of undesirable recessive genes begin to
surface. Inbreeding does not create undesirable trait, it exposes recessive
alleles for hidden weaknesses which are present within the sire and dam.
Because successful inbreeding demands the culling of inferior breeding stock
over many generations (to help eliminate some of the undesirable recessive
genes from the herd), it may not be feasible for some breeders. Not only is
the time factor impractical for most breeders, the intense culling often
necessary may be an economic problem. Additionally, the traits which tend to
surface within the inbred herd (such as depressed growth rate and decreased
size) contrast sharply with what many breeders select for. Therefore, the
breeder must be objective when the need to cull arises.
Perhaps the greatest advantage of inbreeding is that it increases the
prepotency of individuals within a herd and consequently helps to create
distinct true-breeding strains or families. This prepotency (the ability of a
stallion or broodmare to stamp desirable characteristics upon their offspring
with a high degree of predictability) is the result of the parent being
homozygous for important desirable traits. When such a parent carries two
identical alleles on corresponding points of a chromosome pair, he transmits
that allele to the same chromosome point within his offspring. If two such
parents are mated, the offspring will always possess the same desirable trait.
Therefore, as inbreeding increases homozygosity, it also enhances prepotency.
(This is advantageous only if the parents are homozygous for desirable
As mentioned previously, inbreeding exposes certain weaknesses within the
inbred herd. Uncovering these undesirable traits can be an important tool for
the overall improvement within a large breeding program. By setting certain
selection guidelines, and by carefully eliminating inbred individuals which
show inherit weaknesses, the breeder can slowly remove any undesirable
recessive genes from their herd. They will find that vigor and fertility are
actually improved when inbreeding is accompanied by careful selection.
A successful inbreeding program requires good foundation stock and severe
culling over many years. For this reason, inbreeding is usually practiced by
experienced breeders who operate large farms for the production of superior
prepotent breeding stock. It can also be used to establish breeds, or
true-breeding types, with respect to certain characteristics such as color or
A breeding system which uses extreme inbreeding, such as mating between
siblings or between parents and offspring, is referred to as CLOSEBREEDING.
The detrimental effects of inbreeding (such as decrease in vigor, fertility,
athletic ability and size) are usually exaggerated in a closebreeding system.
This is especially true when average breeding stock are used and little
culling has been implemented. Closebreeding can produce extremely good, or
extremely poor, results. Success and failure depend on factors such as
planninig, foundation stock, emphasis on culling, and completeness of pedigree
and performance records, etc. Haphazard closebreeding could be very
detrimental to the overall quality of the resulting offspring. To avoid
disaster, a careful study of the merits and weaknesses of the breeding stock
should precede a closebreeding program. Only the most outstanding mares and
stallions can be used with any degree of safety in a long term closebreeding
Closebreeding is a valuable tool in genetic research, since it quickly
exposes hidden gene types that an individual carries. Because of its extreme
nature and the chance that it may suddenly cause undesirable effects in the
offspring, closebreeding is not often used by horse breeders. Some breeders,
who operate large and well organized program, might utilize closebreeding if
they progeny test their stallions. (One method of progeny testing a sire is to
mate him to a large group of his own daughters. A study of the offspring
determines whether he carries undesirable genes hidden in the heterozygous
state.) After a stallion proves that he is of superior gene type, the
experienced breeder may choose to continue the closebreeding to increase
prepotency of future breeding stock.
LINEBREEDING, the most conservative form of inbreeding, is usually
associated with slower improvement and limited risk of producing undesirable
individuals. It can involve matings between closely or distantly related
horses, but it does not emphasize continuous sire-daughter, dam-son, or
brother-sister matings. The main purpose of linebreeding is to transmit a
large percentage of one outstanding ancestor's genes from generation to
generation without causing an increase in the frequency of undesirable traits
often associated with inbreeding.
Because linebreeding is not based strictly on mating closely related
individuals (with very similar gene types), it does not necessarily cause a
rapid increase in homozygous gene pairs. Consequently, it will not expose
undesirable recessive genes as extensively as closebreeding. For this reason,
linebreeding is generally a safer inbreeding program for most breeders.
Intensive inbreeding (and resulting increased homozygosity) is often
directly related to an increase in the expression of many undesirable traits.
Therefore, the line breeder should carefully study pedigrees for each
prospective mating and determine if, and how closely, the mare and stallion
are related. By following certain guidelines, the breeder can limit inbreeding
(and, therefore, homozygosity) within their herd. At the same time, they may
increase the influence of a common ancestor upon the entire strain or family.
CROSSBREEDING is the mating of horses from different breeds.
Crossbreeding may also be used to produce heterosis, the sudden increase in
vigor and fertility caused by a sudden increase in heterozygosity. Because
horses from separate breeds usually carry very different genotypes,
crossbreeding causes a more extreme form of heterosis. The possibility of each
parent contributing identical alleles to their offspring is remote. Heterosis
from crossbreeding often appears as a sudden improvement in physical
characteristics, such as size, endurance, disease resistance, etc. New breeds
are sometimes established by crossing members of two or more breeds and
carefully inbreeding the original crossbred offspring. Crossbreeding initiates
the desired change, while inbreeding increases the ability of each generation
to breed "true to type".