1965-Voice Of The Tennessee Walking Horse 1965 January Voice | Page 64

Some expected heritabilities for economic traits in
horses are as follows:
Trait
(1)
(2)
(3)
(4)
Skeletal size
Muscularity
Conformation
Temperament and
disposition
(5) Physical
performance
(6) Longevity
(7) Reproductive
performance
Probable Heritability
High
Medium
Low
Medium to low
Medium to low
Medium to low
Low
Some actual heritabilities for quantiative traits
in horses may be lower than those proposed by the
author. There is probably more variation in traits
among horses due to environmental cause than for
other classes of commercial livestock. The horse in­
dustry involves many owners with small numbers
of horses. This likely increases environmental in­
fluences with regard to the overall variation we ob­
serve for nearly all quantitative traits.
The factors that determine the rate of breeding
improvement as a result of selection are: heritability,
selection differential, genetic association among
traits, and generation interval. The higher the heri­
tability of an individual trait, the more rapid is the
expected rate of breeding improvement. Selection
differential is the difference between the selected
individuals and the average of all animals in a herd
from which they were selected. It is influenced by
the proportion of the total selected, the number of
traits selected for, and the prevailing variation in
the initial group. High selection differentials facili­
tate the rate of breeding improvement for a trait.
The greater the number of traits selected for, the
lesser the amount of selection that can be practiced
for an individual trait.
Little is known about genetic associations among
traits in horses. Traits may be independent, positive­
ly associated, or negatively associated. In the event
of no association between two or more traits, the
traits are inherited independently. If genetic associa­
tions are positive, the rate of overall improvement
is facilitated because the association leads to an
automatic improvement of those traits as selection
is practiced for one or a larger proportion of them.
Negative genetic associations hamper or decrease
improvement rates under selection because selection
in the desired direction for one of the related traits
results in a degree of selection in an undesired di­
rection for the other traits. Most quantitative traits
in horses are probably independent; however there
must be some association between racing ability and
conformation.
The generation interval for horses is long, ap­
proximately ten years. This makes the rate of breed­
ing improvement for the species low because of
time required for producing new generations. We
have fewer opportunities to select for the breeding
improvement of horses as compared to most other
classes of livestock.
VARIATION AND SELECTION
Variation is the observable or measurable dif­
ferences between individuals for a given trait. If
there were no variation, all individuals would be
the same. It is the raw material upon which an ani­
mal breeder must base his opinions with regard to
culling or selection procedures for the progressive
breeding improvement of livestock. It has been men­
tioned previously that variation is mainly due to
heredity or environment. An interaction between
these two may be of importance in some instances.
Environment is of particular importance in quan­
titative inheritance.
Selection is of two primary classes, natural and
artificial. Natural selection is that exercised by na­
ture or natural sources and it favors the survival of
the fittest in a particular environment.
Artificial selection is practiced by man and is the
primary topic of this discussion.
Selection involves both qualitative and quantita­
tive traits.
When we select for a single dominant gene we
have the problem of distinguishing between the
homozygous dominant and the heterozygous indivi­
duals because both possess the dominant pheno­
type. The heterozygous individuals must be identi­
fied before they can be discarded. Selection against
a dominant gene is effective and easily achieved. All
we have to do is discard individuals possessing the
dominant phenotype and all carriers of that gene
are removed from a population.
Selection for a recessive gene is simple. All in­
dividuals possessing the recessive phenotype are
homozygous for that gene. Selection against a reces­
sive gene is fairly difficult because heterozygous
individuals, which are the carriers, possess the dom­
inant phenotype. In addition to removing the indivi­
duals possessing the recessive phenotype, we must
identify and eliminate the heterozygotes. This neces­
sitates breeding tests which ar e generally applicable
to sires only.
There are several methods of selection which may
be employed for quantitative traits. In practice, se­
lection by most breeders is a combination of these.
This is true for horses as well as other livestock. Re­
gardless of method, selection is basically employed
to increase the frequency of desirable genes in an
animal population. We are confronted with changes
in popular preferences to some degree and occas­
ionally horse breeders must change their improve­
ment objectives.
Selection based on individuality or individual
phenotypic merit is probably the most prevalent
method of selection in horses and other livestock.
The method is effective for breeding improvement in
the event that the heritabilities of the traits in ques­
tion are high. The effectiveness declines when heri­
tabilities are lower.
Selection is often made on the basis of pedigree
information. In using pedigrees for this purpose,
most weight should be given to the "close ances­
tors. Ancestors removed more than three generations
from an individual have relatively minor influence
(Continued on Pnge 66)
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VOICE of The Tennessee Walking Horse