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THE GENETICS OF THE GRULLA COLOR
A Medium-level Description for Novice Genetics
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This will
not be a technical description, but will be intended more for the person
with a medium interest in learning about the grulla coat color (a beginner or person
early in the learning stage). The grulla color is a rare color (and
difficult to breed for) because the conditions must be "just
right" or else you get another color of foal. In order to have
a grulla foal, you need to have a foal with
 | black legs, |
| a genetically black (non-bay) body, and |
| dun factor. |
There are some pretty basic factors/alleles (we'll call them genes, but
that's not quite correct) that mainly control this:
Before beginning, we need to lay some ground rules to simplify this as
much as possible. For this simplification:
- We are not discussing the effects of the gray gene (or many others) on these base
colors, so for now, just ignore it
- The color "red" also includes yellow as in palomino or dun
or buckskin
- The term "body" includes the barrel, hip, head, and neck,
but not the legs, mane, and tail
- Each gene for a trait consists of a pair of alleles, with one coming
from the sire and one coming from the dam
- We are ignoring a lot of other stuff that will clutter this
explanation up, and that would make it harder to understand
The First Oversimplification
Remember that I am going to oversimplify on this whole
page in an attempt to make this a quick and easy thing to understand, and
that this whole page is arranged according to how my own deranged mind
works. :-) If you go on to read about color genetics, you'll
realize here that some things (mainly vocabulary) aren't quite correct or
are very oversimplified, but my goal wasn't to be 100% correct--it was to
water down the genetics info to make it easier to understand. This
page will actually help you understand genetics and help make you ready
to go to that next level (that page is coming soon). Here are my
first oversimplifications!
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The color of the horse's legs is controlled by the E
and e locus alleles (we'll call it a gene, though that's not quite
correct). E is for black legs, and e is for red legs. One
E will override any e, so any horse with an E
(capital letter) will have black legs. |
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The color of the horse's body is partially controlled by the A
and a locus. An A means red/brown body, and an a means that the body
will match the legs in color. The A is dominant, so any horse
with an A will have a red/brown-based body. |
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The dorsal stripe is controlled by the D and d
alleles. Any horse with a D will be a shade of dun, where a horse
that is dd (no capital D's) will not have dun factor. |
Red Factor
The answer to the question, "Why are his legs red or
black?"
Since grullas have black legs, we know that they must carry at least
one black (non-red) gene. To correctly represent this, we really
need to say that they should not carry two red genes, but instead
should carry one or two non-red (black) genes. The black color is
dominant over the red color, and is represented by the capital letter
E. The red color is represented by the small letter e.
So, in regard to the red genes, there are three possibilities:
| ee, which is a red-bodied horse with red legs |
| Ee, which is a black-legged horse. The body color
depends upon the Agouti locus (below). This horse's foals can
have either red or black legs. |
| EE, which is a black-legged horse that will always throw
black-legged foals (this horse is commonly referred to as homozygous
black) |
Therefore, any horse with black legs must have one "E" or two
"E" genes, and would be either Ee or EE. A red horse would
be ee here. See http://www.vgl.ucdavis.edu/horse/redtest.html
for more details, and information about testing for red factor.
Agouti
The answer to the question, "Why is his body
red or black?"
The Agouti gene is a dominant gene, and if it is present
in a foal in its dominant form, the foal can NOT be grulla (or black, or blue roan). The
Agouti gene mainly affects the color of the body, but not the legs, mane,
or tail. A sorrel horse can have one or two Agouti genes, as can a
bay (their bodies are red). But a black, grullo, or blue roan horse
cannot have any Agouti genes, because they have black bodies. This
is why blacks, grullos, and blue roans are so hard to get, and therefore
are considered a recessive color. The capital letter A represents
the red/brown body caused by the Agouti gene, while the small letter a
represents the lack of the Agouti gene. Agouti possibilities are:
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aa, which is a horse with a body color that
matches its points (legs, mane, tail). There is no Agouti gene
causing the body to be red. The body can be either red or black,
depending upon the red/black genes (e and E) discussed above. |
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Aa, which is a horse whose body will always be
red (never black). This horse can pass along the Agouti (A)
gene, but won't always. |
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AA, which is a horse whose body will always be
red (never black). This horse will ALWAYS pass along an Agouti
gene to its foals, and therefore will never, ever have a true black,
blue roan, or grullo foal. |
To put those two together (red/black and Agouti), you can
get the following combinations:
| Base Color |
Red/Black |
Agouti |
Genotypes |
| Red colors (sorrel, palomino,
etc.) |
ee (no black hairs) |
aa
Aa, or
AA
(all with red legs and bodies bodies) |
ee aa, ee Aa, or
ee AA
(all sorrel/chestnut) |
| Bay colors (bay, buckskin, dun,
etc.) |
Ee or EE |
Aa or
AA
(black legs, red bodies) |
Ee Aa, Ee AA, EE Aa, EE AA (all bay based) |
| Black colors (black, grullo, blue roan, etc.) |
Ee or EE |
aa
(black legs, black bodies) |
Ee aa, or EEaa
(black based) |
Another comment about Agouti genes: The only
ways to know for sure if a horse carries the Agouti (red body) gene are to
have foals from the horse that express the color (bay-based colors), to
have that horse actually expressing the gene (black legs with red/yellow
body), to know the genetics of its parents, or to have him tested for
color genetics (see the bottom of this page). Many people think of
the Agouti gene as being the "bay" gene, because it is what
causes an otherwise black horse to be bay. In reality, the Agouti
gene prevents blacks by turning them into bays. Keep in mind that
sorrel/chestnut colors can carry the Agouti gene, even though they don't
show it. Since their bodies are already red, you just can't tell
they may carry the Agouti gene.
Dun Factor
The answer to the question, "Why does he have a
stripe down his back and dark tips on the backs of his ears?"
Now, the third gene/allele in the series: Dun
factor. This gene causes more than one action on the above
combinations, but in general it creates a distinct dorsal stripe running
fully from the mane and into the tail, and also it dilutes the body color
of the horse to a lighter (diluted) shade. There are other factors
which cause stripes on horses' backs similar to dun stripes, but they are
not true dorsal stripes. Very often, the dun gene will also cause
dark ear tips, leg barring/striping, neck striping, face cobwebbing,
and some other striping. The capital letter D represents dun factor,
and the small letter d represents the lack of dun factor.
The dun locus can be represented as follows:
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dd, or no dun markings (no dun factor) |
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Dd, or dun markings present. This horse
will pass along dun factor markings to about half of his/her
foals. At least one of this horse's parents was a dun-factored
horse. |
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DD, or dun markings present. This horse
will ALWAYS pass along dun markings to his/her foals. Both of
this horse's parents must have carried the dun gene. |
To make the table more complete, we can now add dun
factor:
| Base Color |
Red/Black |
Agouti |
Dun |
| Red colors (sorrel, palomino,
etc.) |
ee (no black hairs) |
aa
Aa, or
AA
(all with red legs and bodies bodies) |
dd, Dd, or DD |
| Bay colors (bay, buckskin, dun,
etc.) |
Ee or EE |
Aa or
AA
(black legs, red bodies) |
dd, Dd, or DD |
| Black colors (black, grullo, blue roan, etc.) |
Ee or EE |
aa
(black legs, black bodies) |
dd, Dd, or DD |
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Now, just put the pieces together. I generally look
first for black legs, and then for body color, and then on and on to
determine the best guess for a horse's genotype. It is a process of
elimination, kind of. For example..... If
a horse is ee Aa dd, it will be a sorrel or chestnut. The ee means
it is a red horse. Aa means it has a red body (A), and
dd means it
does not carry dun factor, so it isn't a red dun, but rather is just a
sorrel. If a horse is ee AA Dd, it will be red dun.
The ee means it is a red horse. AA means it has a red body
(A), and
Dd means it does carry dun factor, so it is a red dun instead of a sorrel.
Going in the other direction, if I see
a horse that is red dun, I know it is ee (because it has no black
legs), and that it has at least one dun allele (because it has a dorsal
stripe). It is not possible to tell if it has an A, because
the A only expresses itself visually if the horse has black legs
and a red body. So, this horse is ee ?? D? for the
placeholders of red factor, Agouti, and dun. The ? marks mean that
I don't know what the horse has at that location in its genetic
structure.
If
a horse is Ee Aa dd, it is bay. E means black legs, so the
Ee is a
black-legged horse. Aa means red body, so this black-legged horse
has a red body. The dd means no dun factor, so the horse does not
become a dun...it stays bay.
Going in the other direction, if I saw
a bay horse, I'd be able to tell certain things about it. First,
it has black legs so I know it has at least one E (but I don't
know for sure if the other is e or E). Secondly, I
know it has at least one A, because it has a red body with black
legs, but I don't know if the other allele there is A or a.
I also know it has no dun factor, so it is dd. Therefore, I
know that for those three traits, the horse is E? A? dd.
If the horse changes slightly,
so that it is aa instead of Aa, then it does NOT have a red body. So
if it is Ee aa dd, it would be black-legged (Ee), no red body (aa), and
dd
means no dun. It's a black-legged, black bodied horse...it's a
black! What if that same horse was a
dun-factored horse? It would be Ee aa Dd, so it would be black
legged (Ee), no red body (aa), and have a dorsal stripe (Dd). What
is a black-legged horse with a dorsal stripe that does not have a red
body? A grulla! The next step is to figure out
what the likelihood of your horse's foal's color will be, using the
genetics of the mare and the stallion!
Links http://www.vgl.ucdavis.edu/~lvmillon/
University of CA Horse Genetics Page Test your foal or
horse...how to be SURE of his/her genetics! Click
here
for the University of California at Davis's genetics test form. Or,
visit
www.petdnaservicesaz.com for an alternative testing site (may be
cheaper). http://www.vgl.ucdavis.edu/horse/redtest.html
Red Factor Test Information http://www.vgl.ucdavis.edu/horse/redfactr.htm
Red Factor Test KIT for sending in samples to have your horses tested
This page last updated
05/22/08 References: Knowledge
gained over the years from books, articles, and acquaintances made vie
e-mail and phone. Dr. Phillip Sponenberg, DVM, Ph D. Equine
Color Genetics. Special thanks to Pam T. of the Everafter
Ranch for helping me proofread this, and keep it correct and yet
simple!
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