1965-Voice Of The Tennessee Walking Horse 1965 January Voice | Page 60

genes are recessive and perpetuated in a population
by heterozygous or carrier individuals. The frequen­
cy of carriers in a population and the frequency of
the lethal gene is expected to be low because of
reasons of natural selection. Since the individual
possessing the lethal trait does not survive to re­
produce, the gene is constantly removed from the
population and its frequency automatically lowered.
Occasionally breeders will unknowingly select
and use a carrier of an undesirable gene such as one
with a lethal effect. Some prominent stallion with
an extensive breeding service record could be invol­
ved in such a circumstance. This "chance" incident
could materially increase the frequency of an und-
sirable gene in a population for an extended period
of time.
It is not the author's intent to imply that lethal
genes are a major threat to the horse or other animal
industries; however, if breeders recognize lethals
when they occur and adopt immediate corrective
procedures in their breeding programs, the problem
can be curtailed in its initial stages. Animal popula­
tions will always have some undesirable genes or
"genetic trash." It is important that the frequency of
such genes remain low.
Some lethal and sublethal genes appear to be
due to incompletely dominant genes or even domin­
ant genes with low penetrance (the gene may fre­
quently fail to show effect or expression). These
are perpetuated in populations by carrier individuals
that survive normally and reproduce. Some sub-
lethals reduce longevity but permit the individual
to reproduce, providing that mortality occurs con­
siderably later than puberty and sexual maturity.
Some lethals have been described in horses. Un­
doubtedly, many have not been identified. Anomal­
ies that result in animal losses should be investigated
thorough ly, especially if the problem is associated
with a particular line or family within any breed.
LethaU in Horse:
(a) Bleeding: Fragile blood vessels of the nasal
mucosa. Observed in Thoroughbred. Recessive
sublethal.
(b) Atresia Coli: Complete or partial closure of as­
cending colon. Recessive lethal.
(c) Epithelio — genesis Imperfecti: Skin imper­
fectly formed on areas of body. Death occurs
a few days following birth. Recessive.
(d) Abracia: Absences of fore limbs. Recessive le­
thal.
(e) Hereditary Foal Ataxia: Periodic failure of
muscular coordination. Final collapse and
death within two weeks. Recessive.
Because lethal and many other undesirable genes
possess low frequencies in a population, related in­
dividuals are more likely to possess the same un­
desirable genes than are unrelated animals. This is
one objection to inbreeding. Breeding plans do not
influence gene frequencies, but do affect gene distri­
bution. Since inbreeding facilitates genetic homozy­
gosity, lethal traits due to recessive genes are likely
to occur among inbreds providing the genes v/ere
present initially. Outbreeding, the mating of un­
related individuals, fends to suppress the incidence
of these same traits.
As mentioned previously, there are many undes­
irable traits in horses influenced by inheritance. A
large portion of these traits are probably quantita­
tive and influenced by environment to at least some
degree. Traits, either good or bad, which tend to ex­
hibit some degree of incidence related to family or
line of breeding apparently are influenced by in­
heritance. The exact, or even approximate, mode of
inheritance may not be known. Some additional un­
desirable traits with probable hereditary cause are
as follows: scrotal and unbilical hernia; parrot
mouth; wryneck; cryptorchidism; unsoundnesses;
vices; and some ailments involving metabolic dis­
orders.
The area of unsoundnesses has always received
considerable attention from horsemen. Very little
can be concluded with reference to hereditary in­
fluences or cause for an individual unsoundness.
Correctness of alignment of feet and legs may be im­
portant because faults may be related to the devel­
opment of some anatomical abnormalities classified
as unsoundnesses. Environment must have an im­
portant role because nutrition and degree of physical
stress through work function certainly have their
effects.
Vices among horses are related to temperament
and environment.
EQUINE QUANTITATIVE TRAITS
In quantitative inheritance, many pairs of alleles
are involved. There is no sharp distinction between
phenotypes, differences being by degree only. Many
traits in farm animals, horses included, which are
of economic importance are of this kind. These traits
are also affected by environment.
Examples of quantitative traits in horses include
skeletal size, muscularity, general conformation,
temperament, and disposition, physical performance
capabilities, longevity, reproductive performance,
and others. It is not possible to discuss or make
reference to quantitative traits in terms of simple
dominance or recessitivity as in the case of qualita­
tive traits because of reasons of genetic complexity.
We can discuss quantitative traits in terms of herita-
bility.
The inherited differences in quantitative traits pro­
vide the basis for selection for improvement through
breeding methods. Very little technical information
is available on heritability estimates for horses.
We can perhaps predict some, in general terms, on
the basis of trends in other classes of livestock.
Heritability is defined as the proportion of the
differences measured or observed between animals
that is transmitted to their offspring. Heritability is
also the correlation between appearance or pheno­
type and heredity or genotype. If phenotype accu­
rately reflects genotype, we will be successful in
planning matings based on phenotypic merit. If
heritability is low, we will make errors in planning
matings based on phenotype because it is not a
good indicator of genotype or "breeding worth." We
need to place emphasis on selection based on family
averages instead of individual merit for traits of low
heritability if progressive improvement is to be
made.
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VOICE of The Tennessee Walking Horse