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AGA38E - Plemenitba koní
Hodnocení materiálu:
Vyučující: doc. Ing. Mgr. CSc. Ivan Majzlík
Zjednodušená ukázka:
Stáhnout celý tento materiálalue from each parent
into the corresponding box within the square.
Each box represents a 25 percent
chance of a specific gene combination. In
the example above, foals from this cross
have a 50 percent chance of getting the Aa
combination and a 50 percent chance of
getting the aa combination.
Stallion (aa) Mated to Mare (Aa)
Stallion
aa
AAaAa
aa
Mare
Understanding genes, simple dominance and the Punnett Square
07CoatColorGenetics 12/14/07 6:51 PM Page 4
The tobiano pattern is under the control
of the dominant gene TO. The most com-
mon genotype for the tobiano is the het-
erozygous TOto. Fifty percent of the foals
produced from a heterozygous tobiano
should have the tobiano pattern.
Fortunately, the homozygous tobiano
TOTO exists. Having two copies of the
tobiano gene does not increase the amount
of white on the horse, but individuals carry-
ing this rare genotype can often be identi-
fied by small dark hairs scattered in clusters
in the white areas of the coat. Not all
homozygous tobianos have these “ink spots”
or “paw prints,” nor do these markings
occur only in homozygous horses. However,
there is a strong association between this
pattern and the TOTO individual.
Homozygous tobianos are beneficial to
breeding programs due to their ability to
produce tobiano offspring. Statistically,
every foal produced from the mating should
receive one copy of the dominant tobiano
gene TO, thus creating a tobiano.
Occasionally, a homozygous tobiano
mating produces a solid-looking horse.
This horse carries the tobiano gene but has
only limited white markings. These horses
are often referred to as minimal-white
tobianos.
For these horses, the tendency of the
tobiano to have a dark head and white legs
holds true. The head may be completely dark
or have very little white on it. However, the
legs will show the specific characteristics.
According to the late Dr. Ann Bowling in
her book Horse Genetics, minimal-white
tobianos have dark spots or streaks in the
white markings extending up from the coro-
net. Also, the white extending over the
hocks ends as a horizontal line.
As of this time, there has not been a case
of a minimal-white tobiano that has failed
to have white leg markings.
Even though these horses are registered
by APHA as solid Paints, they are, in fact,
tobianos and do pass on the dominant
tobiano gene 50 percent of the time.
Homozygous tobianos result from the
mating of two tobiano parents. According
to the predictions of the Punnett Square,
one out of every four foals produced by two
heterozygous tobiano parents should be a
homozygous tobiano.
Some Paint Horse breeders disagree with
the Punnett Square projections, maintain-
ing that the TOTO gene combination is
harder to obtain. They also believe that a
homozygous tobiano is
more likely to be pro-
duced if both tobiano par-
ents come from tobiano
parents.
Currently, there is not a
laboratory test to identify the
tobiano gene. Homozygous
tobianos are identified on the
basis of other genetic evidence.
To qualify as a homozygous
tobiano, a horse must be the
result of a breeding of two
tobiano parents. The horse
should have a characteristic tobiano pattern,
including ink spots, and a pedigree and
breeding record that indicates homozygosi-
ty for the tobiano pattern.
According to the Punnett Square, a
homozgyous tobiano stallion mated to a
heterozygous tobiano mare should produce
50 percent homozygous foals and 50 per-
cent heterozygous foals. No offspring
should be solid.
Determining Tobiano Homozygosity
Breeding a homozygous tobiano should
produce all tobiano foals, with the excep-
tion of a rare minimal-white. A horse that
produces five tobianos out of five solid
mates is thought to have a 97 percent
chance of being homozygous. Seven
tobianos from seven solid partners increas-
es the odds to 99 percent. Ten tobiano off-
spring from 10 solid mates increases the
odds to 99.9 percent.
In addition, genetic marker analysis is
used to try to identify homozygous tobianos.
This analysis is similar to playing the game
Clue. Certain facts are given, and then by the
process of elimination one tries to determine
which parent or parents supplied the
tobiano gene. Remember, the horse must
have been produced from the mating of two
tobianos. Tovero parents do qualify.
The tobiano gene is linked to a gene unit
comprised of the E gene (see page 12), the
Rn gene (see page 17), and two other genes,
ALB and GC, that code for blood proteins.
These four genes lie so close together on the
same chromosome that they are usually
passed on as a unit to the next generation,
making their presence an important clue to
determining a tobiano’s homozygosity.
This fact creates the opportunity to trace
movement of the tobiano gene.
The ALB blood protein comes in two
forms—A or B—while F and S are the sym-
bols given to the two forms of the GC gene.
A major clue to determining tobiano
homozygosity is that 90 percent of the
time, the tobiano gene is associated with
the B form of the AL gene and S form of the
GC gene.
The blood test that is currently used in
the search for the homozygous tobiano
determines which form of the AL and GC
genes a particular horse has.
And don’t forget that the expression of the
E provides a color trace for the tobiano
gene, as well. This means that the tobiano
gene lies close to the gene that determines
whether the base color of the horse is red or
black—e or E, respectively. The goal is to
determine which form of the E gene is
linked to the TO.
(The roan gene, which is in the E Rn ALB
GC unit, appears to commonly exist in the
recessive form Rn+. It is extremely rare to
find a roan tobiano.)
APHA Coat Color Genetics Guide • 5
Homozygous Stallion (TOTO) Mated
to Heterozygous Mare (TOto)
Stallion
TO TO
TO
TOTO TOTO
homozygous homozygous
to
TOto TOto
heterozygous heterozygous
Mare
Breeding the Tobiano Paint
Tobiano
07CoatColorGenetics 12/14/07 6:51 PM Page 5
6 • APHA Coat Color Genetics Guide
Breeding the Splashed White Overo
According to Dr. Bowling’s work, the splashed white pattern is under the con-
trol of a dominant gene identified by the letters Spl. At this point, there has not
been a documented mating of two splashed white overos creating a homozy-
gous individual.
Defining Minimal
Splashed-White
Overo
• A great deal of
white on
the face.
• Solid bodies,
perhaps with a small white spot
on the belly.
• White leg markings.
These horses may be registered as
Solid Paint-Bred, but in reality they are
overos and will produce overos
50 percent of the time.
Breeding the Overo Paint
Little is known about the genetics that create
overo patterns. It is commonly accepted that
overo patterns are under the influence of one or
more dominant genes. It appears that each of the
patterns may be the result of a dominant gene,
but it is also possible that there is only one gene
and that gene modifiers change the pattern.
There is also some evidence that the genes
that produce leg and facial markings may influ-
ence the amount of white on an overo. This
appears to be true for the sabino and splashed
white patterns.
The frame overo, like the tobiano, is thought
to be less sensitive to these genes.
Time and research will eventually answer
these questions. For now, here is what the three
overo patterns seem to have in common.
All the overo patterns have a large range of
expression. At one end, they appear mostly
white. At the other end, a minimal-white overo
may be hard to distinguish from a solid horse.
These minimal-white overos may be the reason
that so many breeders think that overo appears at
random from solid horses. It may also be the rea-
soning behind the belief that it is easier to get an
overo by breeding an overo to a solid that has an
overo parent. These overo breeding stocks may
be minimal-white overos.
The development of a test to determine the
presence of the overo gene(s) will go a long way
toward sorting out this confusion.
The possibility of there being a homozygous
overo does not look good. It is thought that the
homozygous overo is plagued by the lethal
white syndrome. Lethal white foals die shortly
after birth due to lack of proper development
of their digestive systems.
There have been cases of lethal white syn-
drome occurring from the mating of an overo
to a solid. Because of this, the question
becomes: Were these solid horses minimal-
white overos carrying an overo gene and there-
fore producing a homozygous overo foal, or is
there some other gene action associated with
the overo that occasionally produces foals with
the lethal white syndrome?
To confuse the matter further, each overo
pattern has the ability to produce nearly white
normal foals. This is not to say that these foals
may not be plagued by some of the other prob-
lems associated with mostly white horses.
There is some evidence that deafness may
occur more often in nearly white horses. And
with age, the pink skin around the eyes of a
white horse quite often develops cancer.
In spite of the similar action of the hypothe-
sized overo genes, each type of overo has a
characteristic pattern.
Frame overos range from being nearly totally white to the minimal-white indi-
vidual. The minimal-white frame overo characteristically has a lot of white on its
face and a solid body with minimal white leg markings.
Regardless of the expression of the frame overo pattern, these horses pro-
duce overo foals 50 percent of the time. It has been hypothesized that the frame
overo is under the control of a single gene, which is designated Fr.
Frame overos are known to produce lethal white foals. It is not known at this
time whether this condition is created by a homozygous frame overo—FrFr—
but the condition is highly correlated with large amounts of white on the foal.
Defining Minimal-White Frame Overo
• A great deal of white on the face.
• Solid bodies.
• Normal solid horse,
minimal leg markings.
These horses may be
registered as
Solid Paint-Bred, but
in reality they are
overos and will
produce overos
50 percent
of the time.
Splashed White Overo
Breeding the Frame Overo
Frame Overo
07CoatColorGenetics 12/14/07 6:51 PM Page 6
The most interesting thing about Paint
genes is that all four seem to be capable of
combining with each other. The pattern cre-
ated through the action of the tobiano gene
and any overo gene is referred to as tovero.
This name does not indicate which overo
gene is present—just that the overo and
tobiano genes are being expressed in the
same individual.
It is possible that many overo horses have
more than one overo gene creating their
color pattern. The Medicine Hat Paint has
been documented as being produced by
crossing frame overos on tobianos, sabinos
on frame overos and sabinos on tobianos.
From a color breeder’s point of view,
intriguing statistics are derived from horses
carrying more than one copy of a Paint gene.
Ignoring the homozygous tobiano that pro-
duces tobiano foals 99.9 percent of the time,
a horse carrying two separate Paint genes
should produce a spotted offspring 75 per-
cent of the time from solid mates. A horse
having three independent Paint genes is
thought to produce a Paint foal 87.5 percent
of the time. For the horse carrying four Paint
genes, the percentage of spotted foals from
solid partners hits an amazing 94 percent.
So, if the theory is correct and each overo
pattern is under the control of a separate
gene, there is more than one way to increase
the odds of producing a spotted foal.
A Paint with one color pattern gene bred
to a Paint with two Paint color genes pro-
duces a spotted foal 87.5 percent of the
time. If both Paint parents have two Paint
color-pattern genes, the odds of producing
a spotted foal are greater than 99 percent.
The problem is that multiple copies
of Paint genes produce more white on horses,
and some pairings may create lethal white
foals.
Breeding the Tovero
APHA Coat Color Genetics Guide • 7
Breeding the Sabino Overo
Sabino Overo
Tovero
Defining Minimal-White
Sabino Overo
• A great deal of white on the face.
• Possible small roaned areas on the body,
often expressed as a narrow white strip up
a leg or down the throat.
• Normal solid horse, minimal leg markings.
These horses may be registered as Solid
Paint-Bred, but in reality they are overos and
will produce overos 50 percent of the time.
The color pattern referred to as sabino
encompasses a wide range of patterns. It
is possible that more than one pattern has
been included in this category, but at the
moment we are going to assume that this
is the varied expression of a single gene.
This gene has been designated as the
dominant Sb gene.
Lethal whites have occurred from the
mating of two sabinos, but viable white
foals have also been documented. As with
all the overo genes, the wide variation in
pattern leads to a lack of predictability.
The sabino pattern is confusing geneti-
cally. In many, or most, families, it appears
to be transmitted as a polygenic trait
rather than as a single gene. Many horses
appear to transmit it roughly in the per-
centage that they are themselves white.
That is, a sabino Medicine Hat is likely to
produce a higher percentage of spotted
foals (or at least foals registerable as spot-
ted) than is a minimally marked sabino.
Breeding for the sabino pattern has a
few interesting quirks. In many breeds it is
desirable to have flashy white marks, but
not body spots. This includes the
Clydesdale, Shire, Welsh Pony, Arabian
and even the Quarter Horse (for some
breeders).
Clydesdale breeders especially like the
white marks, but most prefer a bay body
color. The general rule that many
Clydesdale breeders use is to mate horses
with four white feet (and usually roany
bodies, resulting from the sabino pattern)
to horses with one dark foot. This tends
to result in the mating of horses with too
much sabino expression to those with too
little expression. On average, the resulting
foals come out with fairly
minimal expression. In the
Clydesdale, this means
few body spots and rel-
atively few roans, which
pleases most breeders and buyers of this
breed. They still do get the occasional
Medicine Hat, though.
Some horses that lack body spots, but
that have the high white socks that creep
up toward the body, are indeed sabinos
and can be useful in Paint breeding pro-
grams. The sabino pattern is a great pre-
tender, but is also responsible for some
very attractively marked horses.
The sabino pattern is probably the most
common “cropout” from the Quarter
Horse and Thoroughbred breeds. In many
cases, an investigation of the cropout’s
parents reveals horses with extensive
white markings. In reality, these parents
are probably minimally marked sabinos,
which occasionally produce foals with
more sabino expression than themselves.
A few cropouts, including nearly white
ones, have very dark parents, or even par-
ents with no white marks. These parents
are clearly not sabinos, and demonstrate
that there may be mechanisms that can
mask the expression of the sabino pattern.
If the sabino pattern is merely an exten-
sion of “normal white marks,” then this
means that an occasional solid-colored
horse (with no white marks) may be able
to mask both white marks and the sabino
pattern. The practical consequence of this
is that such horses make poor choices for
an outcross breeding program because
they can decrease the percentage of spot-
ted foals.
07CoatColorGenetics 12/14/07 6:51 PM Page 7
8 • APHA Coat Color Genetics Guide
A good horse is a good horse, regardless
of its color. Yet, color can be a major asset
when a horse is for sale, and it can make a
difference in the amount of attention a horse
gets in the show ring.
In addition to sporting various patterns
of white patches—expressed as tobiano,
frame overo, sabino, splashed white or
tovero—every Paint Horse also has a back-
ground color.
Coat patterns have many background col-
ors, and controlling them genetically can be
complicated. Anyone wishing to breed for
specific background colors has an interest-
ing challenge before them. The breeder
must combine specific colors with specific
Paint spotting patterns, which requires care-
ful planning and a knowledge of genetics.
Some Paint breeders prefer darker back-
ground colors, such as bay, chestnut and
black, over the lighter colors such as dun,
palomino, grullo or buckskin. The reason
for this preference is because the contrast
between the white Paint patterns and the
darker base colors shows up better than it
does with the lighter colors.
This rule, though, is not absolute, and the
light-colored duns, grullos, buckskins and
palominos are popular among many breed-
ers. Taste in color is an individual preference.
Perhaps it is because of this variety of
preferences that coat color genetics is one of
the few areas of equine genetics where sci-
entists have been able to develop sophisti-
cated theories about how specific genes
determine the color of a horse’s hair.
However, it is important to realize that
much of the theory of coat color genetics is
just that—theory. At this point in time, only
the presence of four color genes can be con-
firmed in the laboratory: the tobiano gene,
the recessive form of a gene that creates a
red horse, the cream gene and the agouti
(bay/black).
The action of the rest of the color genes is
purely hypothetical. Because of this, theo-
ries may change as more tests become avail-
able to identify specific genes.
Identifying coat colors can also be confus-
ing. There is a tremendous range of shades
within a color, and different types of color
without recognized names. There are also
coat colors that appear to be identical but are
under the influence of different genes.
Breed associations have also contributed
to some of the confusion. Thoroughbreds
registered by the Jockey Club are called
roans if they have a red body with white
hairs. According to their definitions, grays
are dark horses that are graying. Technically,
whenever the gray gene is present, the horse
is gray regardless of basic coat color. Horses
of any color (with the exception of true
white) can gray.
The American Quarter Horse Association,
trying to keep up with the current coat color
theory, has changed the description of a
“buckskin.” In the past, a buckskin was any
canvas-colored horse with black points. It
could have zebra markings and a line-back
and still be a buckskin. Today, all line-
backed horses with zebra markings are
referred to as duns by AQHA, unless they
are sorrel or chestnut duns. These are called
red duns. Black duns are called grullos.
APHA’s color criteria is the most descrip-
tive of the three associations when it comes
to roan, giving roan three basic colors: black
(blue), bay and red. Starting in 2000, it
became possible to register bay roans, while
the term red roan designates sorrel/chestnut
roans.
If all of this sounds confusing, take heart.
It is possible to stack the deck in your favor
when trying to produce a specific-colored
offspring—if you understand the underly-
ing genes that create the colors.
Coat Colors
07CoatColorGenetics 12/14/07 6:52 PM Page 8
APHA Coat Color Genetics Guide • 9
The Basic Rules of
Coat Color Genetics
While it is true that the control of color is complicat-
ed, it is also true that the lighter colors are all dominant
to the darker ones. This general rule is oversimplified,
but it works in most cases.
Therefore, the light colors do not pop out—except
rarely—as surprises. That is, you have to breed to a
light color to get a foal of a light color. This fact has
some consequences for Paint breeders.
If the breeder prefers the darker base colors, then it
is important to always select the darker colors for their
breeding programs. This is especially true if outcross-
es are sought, because the lighter colors are fairly com-
mon in the Quarter Horse. They are present, but rare,
in the Thoroughbred.
On the other hand, if the light base colors are desired,
then it is important for the breeder to always include at
least one light-colored parent in matings in order to
boost the chances of producing a light-colored foal.
The downside of using two light-colored horses in a
cross (specifically palominos and buckskins) is the
occasional production of cream-colored horses—the
cremello and perlino. These horses are nearly white,
and it is difficult to see the contrast between any Paint
spotting and the pale background color.
The line-backed dun colors only rarely can produce
a cremello foal, making them safer to mate to other
light colors because cream foals occur in such mat-
ings infrequently.
The darker colors, usually considered to include bay,
chestnut and black, are easier for most breeding pro-
grams. These have a peculiar interaction in that chest-
nut (and sorrel) are recessive to bay and black, but act
to cover them up. This means that it is impossible to tell
just from looking whether a chestnut or sorrel horse has
the genetic makeup to produce black or bay. Testing for
the Agouti gene is helpful.
Reviewing the rules
To review, the basic rules for producing colors are:
• It usually takes at least one light-colored parent to
produce a light-colored foal.
• Chestnut and sorrel, when mated to one another, can
produce onl
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