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. Nicks 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 foal.
If 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.
*** 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.
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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)