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© Diatom Graphics
in light of genetics.
by Ann T. Bowling PhD ©1995
Newcomers to horse breeding often look for pedigree formulas or hope to emulate a particular breeder's program by using related stock. Unfortunately for novices, the truths of horse breeding are that many successful horse breeding judgments are in equal measure luck and intuition. Horse breeding is not as easy to fit to formulas as breeding for meat or milk production. Many of the highly valued traits of horses such as breed type or way-of-going are subjectively evaluated in show ring events. Winners may reflect the skills and show ring savvy of the trainer/handler, as much as the innate abilities of the horse. Some breeders can learn to predict to their satisfaction the approximate phenotype to expect from a selected mating because of their years of experience studying horses and their pedigrees, but their skill cannot always be taught to others and may not work with unfamiliar pedigrees.
Horses considered to be of excellent quality
often present a pattern of recurring pedigree elements. Breeders
naturally seek to define pedigree formulas or "nicks" to design matings
that will consistently replicate this quality. But breeding horses is not
like following a recipe to make a cake. You cannot precisely measure or
direct the ingredients (genes) of the pedigree mixture as you can the flour,
sugar, chocolate, eggs and baking powder for a cake. You can construct pedigrees
to look very similar on paper, but the individuals described by those pedigrees
may be phenotypically (and genetically) quite different. Before seriously
considering any breeding formula scheme it is essential that breeders understand
the most basic lesson of genetics: each mating will produce a genetically
different individual with a new combination of genes.
Basing a program on champions
Novice breeders are often counseled
to "start with a good mare." This seems to be reasonable
advice, but does not make it clear that the critical point is to learn to
recognize a good mare. Sometimes breeders fail to produce a foal that matches
the quality of its excellent dam, while less impressive mares in other programs
produce successfully. Probably the lack of objective criteria to evaluate
horses accounts for both observations. A "good mare" need not be a champion,
and a champion is not guaranteed by dint of show ribbons to be a "good mare."
As well, we do not know the inheritance patterns of highly valued traits
for show ring excellence. If the ideal type is generated by heterozygosity
(for example, the ever useful example of palomino), the only infallible
way to produce foals that meet the criterion of excellence (palomino color)
is to use parents of less desirable type (chestnuts bred to cremellos).
This example is not to be taken as a general license to use horses of inferior
quality, but to provoke critical thinking about the adequacy of general
breeding formulas to guide specific programs.
A master breeder needs several generations (generation interval of horses is estimated to be 9-11 years) to create a pool of stock that contains the genetic elements that he or she considers important for the program vision. To learn to identify essential characteristics, a breeder needs to evaluate the horses and their pedigrees, not advertisements or pictures. When a breeder discovers those elements, he or she can make empirical judgments and is on obvious path for making good breeding decisions.
The cult of the dominant sire
In some circles, the highest praise
of a breeding stallion is that he is a dominant sire. Another
widely encountered livestock breeding term for an elite sire is prepotency.
The implication is that all his foals are stamped with his likeness, regardless
of what mare is used. This concept would appear to contradict the advice
"start with a good mare." Those owners who strongly believe in the strengths
and qualities of their breeding females would surely question the value
of a so-called dominant sire who could seemingly obliterate valued characteristics
that would be contributed by their mares. A good understanding of genetics
should allow a breeder to put the proper frame of reference to terms such
as dominance and prepotency as applied to breeding horses. Some animals
transmit certain characteristics at a higher frequency than is generally
encountered with other breeding animals. Coat color is always the conspicuous
example. Any stallion whose offspring always or nearly always match his
color is popularly described as a dominant sire. To be excruciatingly correct,
for at least some of the effects being considered the genetic interaction
is not dominance but epitasis and homozygosity. A stallion could be homozygous
for gray, leopard spotting or tobiano, so that every foal, regardless of
the color of the mare (with the possible exception of white), would have
those traits. Homozygosity for color is not necessarily linked with transmission
of genes for good hoof structure, bone alignment in front legs, shoulder
angulation or other traits that may be desirable. Most conformation traits
seem to be influenced by more than one gene. Some stallions may be exceptionally
consistent sires of good conformational qualities, but it is unlikely that
every foal will have these traits or that any stallion could be so characterized
for more than a few traits. The balanced view is that a battery of stallions
is needed to meet the particular genetic requirements of each of the various
mares in the breed. No one stallion can be the perfect sire for every mare's
assays for genes important for program goals are available, the probability
of obtaining foals with selected traits from specific breeding pairs can
be predicted. For many horse coat colors, offspring colors can be predicted,
but conformation and performance traits are not well enough defined for
predictive values to be assigned. So little is known about the genetics
of desirable traits, it is premature to suggest that any general technique
of structuring pedigrees consistently produces either better or worse stock.
BINT SAHARA was a legendary producer of champions (this one is ROYAL GREY). What was her secret? Pedigree? Genes? A magic nick? Some of the above with a touch of just plain luck? (Photo: Linda Sale-Paich)
An excerpt, printed by permission of the publisher, from the forthcoming book Horse Genetics by Ann T. Bowling. Publication by CAB International is anticipated in early 1996.