Download 08 Horse Genetics

Horse Genetics
Equine Science & Technology
Horse Genetics
Nature ordained that genetics be applied to horse breeding
long before there were geneticists. Prior to the
domestication of horses, there was natural selection for
speed and stamina. One of the most important defense
mechanisms of the horse was to outrun its enemies.
 Natural selection was probably very effective in improving
speed and endurance, since the slower horses were
eliminated by predators.
Horse Genetics
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Many horse breeders have been practicing genetics as they
concern themselves with the are of breeding. Their guiding
concept of heredity is that “like begets like”.
Breeders in the 18th century made a tremendous
contribution pointing the way toward horse improvement
before Mendel’s laws became known to the world.
As knowledge of genetics developed, there evolved an
understanding of the science that underlies the art of horse
breeding.
Horse Genetics
Genetics
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The bodies of animals are made up of millions or even
billions of microscopic cells.
Each cell contains a nucleus in which there are a number of
pairs of bundles called chromosomes.
In turn, the chromosomes carry pairs of minute particles
called genes, which are the basic hereditary material.
The nucleus of each cell of a horse contains
32 pairs of chromosomes, or a total of 64.
Horse Genetics
Genetics
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There are thousands of pairs of genes on each
chromosome.
Genes determine all the hereditary characteristics of
animals from the body type to the color of the hair.
The mule has an uneven number of chromosomes and
cannot reproduce.
The genotype of an animal refers to the specific genes it
possesses on its chromosomes.
Genome describes the complete set of “instructions” for
making an organism.
Horse Genetics
Genetics
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Dominance gene expression has a full allele effect. An
example would be when a gray horse produces a gray
horse.
Recessive character appears only when both members of a
pair of alleles are alike.
The job of transmitting qualities from one generation to the
next is performed by the gene cells- a sperm from the male
and an ovum, or egg from the female.
Horse Genetics
Genetics
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There are alternate forms of each gene. The alternate gene
forms are referred to as alleles. If the alleles are the same,
the condition is referred to as homozygous. If the alleles
are different, the condition is called heterozygous.
Selection and crossbreeding are the tools through which the
horse breeder may obtain stallions and mares whose
chromosomes and genes contain similar hereditary
determiners-animals that are genetically more
homozygous.
Horse Genetics
Genetics
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Gene changes are known as mutations. A mutation may
be defined as a sudden variation that is later passed on
through inheritance and that results from changes in a
gene or genes.
Horse Genetics
Simple Gene Inheritance (Qualitative Traits)
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In the simplest type of inheritance only one pair of genes is
involved. Thus, a pair of genes may be responsible for the
color of body hair in horses.
The idea that certain basic colors may have a rather simple
explanation of inheritance should not alter the fact that
other genes may play an important role through their
influence on basic schemes.
The possible gene combinations are governed by the laws
of chance.
Horse Genetics
Simple Gene Inheritance (Qualitative Traits)
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Other possible examples of simple gene inheritance in
horses might include eye color and the set of the ears on
the head.
Horse Genetics
Dominant and Recessive Alleles
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A dominant allele may cover up a recessive allele.
Therefore, a horse’s breeding performance cannot be
recognized by its phenotype (how it looks).
Recessive genes can be passed on from generation to
generation, appearing only when two animals, both of
which carry the same recessive allele, happen to mate.
Horse Genetics
Dominant and Recessive Alleles
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Reputable breeders have an obligation, not only to
themselves, but to their customers. Purebred animals
must be purged of undesirable genes by eliminating those
stallions and mares that are known to have transmitted the
undesirable recessive gene.
Horse Genetics
Incomplete Dominance
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In some cases, dominance is neither complete nor absent,
but incomplete or partial and expresses in a variety of
ways.
Perhaps the best known case of this type in horses is the
Palomino color. Genetic studies of the Palomino indicate
that the color is probably unfixable- that it cannot be made
true breeding, no matter how long or how persistent the
effort.
Horse Genetics
Multiple Gene Inheritance (Quantitative Traits)
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Important characters such as speed are due to many genes;
thus, they are called multiple-gene characters.
Quantitative inheritance refers to the degree to which a
characteristic is inherited.
For example, all Thoroughbred horses can run and all
inherit some ability to run, but it is the degree to which
they inherit the ability that is important.
In quantitative inheritance, the extremes (either good or
bad) tend to swing back to the average.
Horse Genetics
Horse Color
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Recent advances in genetics have allowed us to
understand better how coat color is inherited, thus making
it possible to plan matings that will produce foals of a
certain color.
Horse Genetics
Gene A
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Genetic control of black color restricted to the points of a
horse is through gene A, or the A allele.
The A allele is dominant.
A uniformly black horse has the recessive genotype aa.
Horse Genetics
Gene E
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Gene E, or the E allele, controls the expression of black
hair on the body.
The recessive form ee, results in uniformly red-colored
horses.
The E gene is related to (and can mask) the A gene.
Horse Genetics
Gene G
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Gray coloring is due to the dominant gene G.
Any horses with the G allele will be gray; non-gray horses
will have the genotype gg.
Gray horses can be born any color.
Horse Genetics
Gene W
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Solid white coloring is controlled by gene W.
The dominant homozygous (WW) condition is lethal; thus,
all white horses have the genotype.
All non-white horses have the genotype ww.
Horse Genetics
Gene C and D
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The C and D alleles are dilution alleles. Gene C affects
only red pigment and causes the pigment to lighten.
The C gene does not affect black pigment.
Gene D differs from gene C in that it can dilute both black
and red pigment.
With the D allele, black is diluted to a mouse or slate grey,
and red is diluted to a yellow-tan.
Horses with the D allele also show primitive marks such as
the dorsal stripe, shoulder stripes, and leg barring.
Horse Genetics
Gene TO
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The spotting of paint and pinto horses is controlled by the
tobiano gene TO.
The tobiano color pattern is dominant; thus, overos must
have the genotype toto.
Homozygous TOTO horses are rare and are sought after by
horse breeders for their ability to always produce spotted
foals.
Genetic tests are available to determine whether a horse is
homozygous.
Horse Genetics
Heritability of Performance Traits
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Relatively little scientific work has been done on the
heritability of performance of horses- on the genetics of
working ability, racing ability, cutting ability, etc.
The horse is the last farm animal to which the science of
genetics has been added to the art of breeding.
Differences in the performance ability (working, racing,
jumping) are due to two major forces-heredity and
environment.
Horse Genetics
Heritability of Performance Traits
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The important environmental factors in determining the
overall performance of horses are nutrition (both prenatal
and postnatal), health care, quality of training, ability of the
handler, and injuries.
An important genetic principle is that traits as such are not
inherited. The ability to respond to a given set of
environmental conditions in order to produce a trait with a
measurable effect is inherited.
Reliable estimates on the heritability of performance traits
in horses are limited in comparison with those for other
species.
Horse Genetics
The following conditions would tend to indicate a
hereditary defect in horses:
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If the defect had previously been reported as hereditary in
the same breed.
If it occurred more frequently within certain families or
when there had been inbreeding.
If it occurred in more than one season and when different
rations had been fed.
Horse Genetics
The following conditions might be accepted as indications that
the abnormality was due to a nutritional deficiency:
 If it had previously been reliably reported to be due to a
nutritional deficiency.
 If it appeared to be restricted to a certain area.
 If it occurred when the ration of the dam was known to be
deficient.
 If it disappeared when an improved ration was fed.