Equine Coat Color Genetics
Lesson Three
The Cream Dilution
Palomino, Buckskin,
Smoky Brown, Smoky Black, Cremello, Perlino, Brown Cream & Smoky Cream
The Single & Double
Cream Dilutes
The term “dilute” is used to describe
multiple colors: Cream, Champagne, Dun, Silver and Pearl. In recent years, equine studies have shown
that each of the colors are actually separate
genes. With the exception of the pearl
gene, they reside on separate sections (alleles) of the chromosome. In this section, we will address the Cream
gene only.
This is a good place to add two more
words to our genetic vocabulary. The color of a horse determined by the way it
looks is its phenotype. The actual genetic code of the individual is
its genotype.
Determining the genotype has been
described as "detective work". While there are laboratory tests
available for most known color genes at this time, in the past, examination of the
color of the ancestors and the colors produced from specific crosses, and
tracing the genes from generation to generation, was the only way to
ultimately determine the genotype of the horse in question. However, almost all of the results obtained
by the "old method" (which is still widely used) have been confirmed by the lab tests
now available.
A Word About "Albino"
An albinos is
by definition “A person or animal lacking normal coloring in the eyes, hair,
and skin due to a hereditary inability to produce the skin pigment melanin. The
condition itself is called albinism.”
(http://medical-dictionary.thefreedictionary.com/albino).
Mice carry this gene, which in mice produces an all-white mouse with red
eyes (from the blood vessels showing through the clear, unpigmented
iris) and pink skin, completely devoid of any color pigment. Studies have shown that the Equid species (all horses, mules, donkeys, asses, zebras,
etc) do not carry the genetic mechanism to produce an albino. Remember, "albino"
in most animals denotes complete, total absence of color producing
genetics. So a horse with a white coat,
pink skin & blue eyes is not a true "albino". The effect described above can be produced by
any of several combinations of color-modifying genes. Most striking is that of a
multi/maximum-pinto -- think of the horse being covered by so many white spots
that there is no color left showing through anywhere -- produced by any number
of pinto (aka paint) genes.
The Cream gene - Single and Double Dilutes
The most common dilution gene, the
cream color-modifying gene (Cr), is responsible for palomino, buckskin, smoky brown and smoky black
colors, as well as what were (and sometimes still are) simply called “double
dilutes” (cream dilution implied).
Cream is an incompletely dominant gene. An easy way to think of the concept of
“incomplete dominance” is to think of the gene providing only part of its
capability to dilute when present in one copy, and its full capability when
present in two copies. In other words,
an incomplete dominant expresses itself in a stronger form when two (2) copies
are present (when homozygous).
Because it is a type of dominant gene,
we will use the abbreviation beginning with an upper-case letter
: Cr. The abbreviation C is not used because in
other species, such as mice, it represents the absence of the albino gene
(being, in those cases, the abbreviation for "color".)
The absence of the cream gene, or
not-cream, will be abbreviated cr.
Single
and Double Cream Dilutes
When present in one copy
(heterozygous), the cream gene changes the red body color of sorrels/chestnuts
and bays to a golden or tan color. It
changes a sorrel/chestnut to palomino, a bay horse to buckskin, a brown horse
to a smoky brown (sometimes called a brown buckskin), and a solid black horse
to a smoky black. On a solid black
horse, a single cream gene can be evinced by red or tawny highlights, or it may
be virtually invisible.
All of these single-cream-gene colors
used to be called, simply, “single dilutes”, because cream was the only
dilution gene known until recent years.
When present in two (2) copies, or
homozygous, the incomplete dominant intensifies and changes the single-dilute
colors further. Palominos become cremellos, buckskins become perlinos,
smoky browns become brown creams, and smoky blacks become smoky creams.
Some breeds of horses do not carry the
cream gene (Cr). The two which come to
mind immediately are the Friesian and the modern Arabian. In purebred horses of these breeds, one will
never see palomino, buckskin, smoky brown, or smoky
black animals.
For a quick, fun introduction to the
cream dilutes, see this web page: http://www.horsecolors.us/dilutions/cream/javatest2.htm
Palomino and Cremello
In the heterozygous form (Crcr) this gene will change a
sorrel/chestnut color to gold and the mane and tail to white. This gives us Palomino.
Thus, the palomino horse is a chestnut horse (ee) with one copy of the dominant Cr gene. The genetic expression of this would be ee (any agouti) Crcr/
The ideal shade of palomino is a body
the color of a newly minted gold coin with a white mane and tail. However, the body color of palominos can
range from smoky gray to a yellow cream color. The manes and tails can be white
or ivory or, in extremely rare cases, a shade close to or even darker than
their body color. (See
"Sootiness")
When the Cream gene is present in the homozygous form (CrCr) all of these coat colors lighten further. One can compare it to putting one creamer or two in a cup of coffee.
The palomino coat
color will lighten one more step to cremello, and the
normal brown eye will change to a blue eye as well. The genetic expression of
this “ee (any agouti) CrCr”.
Palomino Coloring Cremello Coloring
The physical traits of a cremello are a cream colored body coat, a lighter/white
colored mane and tail, a warm pink skin tone with some dark specks here and
there, and blue to blue-green eyes. Blue eyes on a double dilute are not the result of a blue-eyed
gene, but of dilution of the iris pigment, and when one breeds a double cream
to a non-cream the resulting (single-dilute) foal will not have blue eyes,
unless the other parent carries the other genetics for blue eyes.
The way most cremellos
occurred in the past was through the mating of two palominos - ee (any agouti) Crcr. This gives a 25% chance of
sorrel/chestnut, 50% chance of palomino and 25% chance of producing a cremello. Before cremellos were eligible for registration with the AQHA,
that was the only way to breed for palomino in that breed, with a 25% chance of
producing a "non-QH" from two pure QH parents!
If one breeds a cremello
to cremello one will always get a cremello. The effect of a double dose of the cream gene
is not magnified and there is no increase in the full effect of the cream gene
in homozygous form. Each parent gives
one Cream gene to the offspring insuring the resulting foal inherits two cream
genes. Remember, the maximum number of
genes involved are two (homozygous state) and the genes will never ‘magnify’
nor mutate when two double-dilute horses are bred together.
Buckskin
and Perlino
Buckskin is the result of one cream
gene on a bay horse (black plus Agouti - E_A_). The single cream gene changes the reddish body
color of a bay horse to a golden or tan color, but does not affect the black
mane, tail, ear tips, or legs of the horse.
Typical Bay Typical Buckskin
When two cream genes are present, the
black coloration of the mane and tail are lightened to a pale gold color and
eyes are changed from normal brown to blue.
The most common way to produce Perlino is
through the mating of two buckskins - E_A_Ccr. This will give a 25% chance of a bay,
50% chance of a buckskin and 25% chance of a perlino.
Typical Buckskin Typical
Perlino
Smoky Brown and Brown Cream
Smoky brown is the result of one cream
gene on a brown (E_ plus AtAt or Ata) horse. Though some still refuse to accept the brown
color group as extant, there are genetic tests as well as visible clues that
differentiate it from black or bay.
A
smoky brown may also be referred to as a brown
buckskin. This is a brown horse (see
description in previous lesson) with one cream gene. As one might expect, the result is somewhere
between buckskin and smoky black. Some
simply call these "very sooty buckskins", but when the genetic
testing is performed, the difference is clear.
They will have extensive darkness throughout the body when compared to a true, bay-based buckskin, yet golden-tan areas much more
extensive than on a smoky black. The
points remain black. The areas that are
light on an undiluted brown horse are lighter on a smoky brown (muzzle, flanks,
armpits, belly, and sometimes "eyebrows".) The genotype for this color would be E_,
AtAt or Ata, Crcr.
Photo from horsecolorforum
When another cream gene is added, the
resulting color is called brown cream, and looks much like a perlino, but usually with a slightly darker body
color. We have requested verified photos
of these colors, which will be included as soon as they are received.
Smoky Black
Smoky back is the result of one cream
gene on a black horse. The single cream
gene does not dilute the black color of the horse in the same way it dilutes
red body color. Sometimes there will
only be reddish or tawny highlights on the black body; sometimes the mane will
"sunburn" to red much more quickly than the mane of a typical non-cream
black; and sometimes it will seem impossible to detect, with the naked eye,
that there is a cream gene present in the horse's make-up.
A
black Friesian (no cream gene).
(Many Friesians have a lot of brownish coloring, but they all genetically test
pure, solid black -- no cream. We are
currently investigating this and other instances of "light black" as
a separate phenomenon.)
A black mare which produced a
buckskin when bred to a bay stallion,
indicating that she is a "smoky black", so her slightly off-black color
is caused by the action of one cream gene on solid black. (E_aaCrcr)
When two cream genes are present, the
black coloration of the mane and tail are lightened to a pale gold or orange
color, and the eyes are changed from normal brown to blue.
Smoky
Black Smoky Cream
When two copies of the cream gene are present, the cream gene reaches full affect. The effect of the cream gene on black horses may be the most difficult to understand, as when one copy of the Cream gene is present there may not appear to be any change to color, but when two copies are present the change is dramatic. Usually the body color of a smoky cream is a bit darker than with the perlino or cremello, which would be expected, since the black body color is being doubly diluted rather than the red.
Also see
http://www.horsecolors.us/dilutions/cream/cream_chart.htm
and
http://www.horsecolors.us/dilutions/cream/cream_foal_chart.htm
... for more insight into the cream colors and their
inheritance. The second chart was
created before the brown (At) gene was isolated
.