The Genetics Of Spots - Dilutes Australia Ltd

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Lp and PATN - the genetics of spots
Leopard Complex spotting (Lp), the result of an incompletely dominant mutation in TRPM1, produces a
collection of unique pigmentation patterns in several breeds of horse. While the Lp mutation allows for
expression of the various patterns, other loci are responsible for modification of the extent of patterning.
Pedigree analysis of families segregating for high levels of patterning (80-100% white hair coat) indicated
a single dominant gene (PATN1) as a major effect modifier for Lp.
Lp is the gene that switches on or gives the Appaloosa its visual characteristics, of which there are 4:
•
white sclera in the eye
•
mottled skin
•
striped feet
•
varnish or Lp Roan which is not to be confused with Classic Roan
Lp is incompletely dominant. Two recessive copies (lplp) gives you a solid or non spotty horse. With
one dominant copy and one recessive (heterozygous or Lp/lp) you will get polka dots in the coat. And with
2 dominant copies you will get a horse with no or very few dots in their white pattern. These are called
snowcaps when there is only a white area on the rump/torso, and fewspots or near fewspots where there's white
on over 80% of the horse's body.
The second part of the spotted ‘coat colour’ equation are the pattern genes, dubbed PATN for short. These
are inherited separately to Lp and are responsible for creating the areas of solid white patterning. This
patterning appears to be a polygenic or ‘many gene’ trait in that the extent of patterning varies according to
which of the PATN genes, and how many, the horse has - some types of PATN appear to produce large
amounts of patterning whilst others only produce minimal amounts. The type that produces the most white
is referred to as PATN1.
Lp and PATN are inherited separately so a horse can inherit PATN on its own, Lp on its own, or both Lp
and PATN together. A horse that carries Lp on its own may exhibit any or all of the Leopard complex
characteristics but will not have the areas of solid white patterning because it doesn’t carry any of the PATN genes.
A horse that has inherited both Lp and one or more of the PATN genes displays some amount of white patterning at birth, the
extent of which is controlled by which, and how many, of the PATN genes it carries.
If only PATN genes are inherited (no Lp) the horse will present as a solid coloured horse because Lp is the gene that "lights
up" the appaloosa patterns ie PATN requires Lp to become visible - no Lp = no visible spots, no white patterning and no
characteristics. However, if this horse is put to a horse that carries Lp and no PATN and the resulting offspring inherits both Lp
and PATN it will display coat patterns not seen in either parent.
If only Lp is inherited the horse may present with any or all of the visual characteristics listed above. The most visible
version of Lp is varnish - the photos on the right
clearly show the varnish effects of Lp on this mare
who was born solid but is now completely varnished out as an adult. She has pretty much all the
characteristic darker areas remaining at bony points
(on the face, usually in a distinct V on the bridge
of the nose; on the cheeks, point of shoulder, elbows, knees, point of hips, hock). She also has a
few leopard spots which have become visible as
she varnished out. It is thought that there are differences between homozygous and heterozygous
Lp horses relating to how quickly they varnish
out and so on but I don’t know if this has been
proven as yet. This mare is heterozygous for Lp as
her dam is a Thoroughbred.
A horse that inherits both Lp and PATN will exhibit not only the various visual characteristics of Lp but will also have white
coat patterns. The size of these patterns will vary according to which PATN genes it has; they can range from just a sprinkling
of white over the hips (snowflake) to the extensive white, referred to as PATN1 (or leopard pattern), seen in the leopard and
fewspot.
The expression of PATN in conjunction with 2 copies of Lp (homozygous Lp so
Lp/Lp PATN#/__) can be anywhere from 50% - 100% which creates the snowcap
(blanket with no spots inside the blanket) and fewspot coat patterns. These horses
will always pass on one copy of Lp to their offspring.
The fewspot is the maximum expression of Lp and PATN; it creates a pure white
horse that has a few base coloured spots in the coat, generally on the flank, elbow,
neck and head. They can also display varnish marks. The genetics of a fewspot is
believed to be Lp/Lp PATN1/? ie homozygous Lp with at least one copy of the all
over body white version of PATN - PATN1.
Fewspot
Kaleidoscope (Vol 55 - Page 23)
The other homozygous Lp pattern is the snowcap. The snowcap differs
from the normal blanket pattern in that the blanket does not have spots or
colour within its extent courtesy of the 2 copies of Lp. The blanket can vary
in size and can extend across most of the body with the base colour being
retained on head, legs, flanks and elbows. The snowcap probably carries
another version of PATN which restricts the white patterning to a blanket Lp/Lp PATN/?) rather than the all over body white PATN1.
Snowcap with
varnish
The Leopard is the other pattern that carries PATN1 or all over body
white but instead of being
homozygous for Lp, they
are heterozygous (Lp/lp
PATN1/?). The single
copy of Lp allows a greater expression of colour or ‘spots’ which are
often also larger than those seen on
a fewspot. A leopard is basically a
white horse with dark spots in its
base colour that cover the entire
body. A variation of the leopard
is the Near Leopard, a pattern that
exhibits extensive leopard type
spotting over most of the body but
retains some solid colouring (varnish marks) on the knees, chest, flanks, underside of the neck and cheeks as illustrated by these two palouse ponies.
Small blanket
The popular Blanket pattern has a
white blanket with dark spots within the
white. The spots are the same colour as the
horse's base colour. The blanket can range
from just a small area over the hips to one
that covers most of the back from the withers back. A horse with a spotted blanket
pattern is heterozygous for Lp and carries
the same type of PATN as the snowcap ie
the version that restricts the white pattern
to an area on the back, hips and rump (Lp/
lp PATN/?)
Larger blanket
expression may
be due to the red
base of the palomino colour.
The Snowflake pattern is white
spotting and/or flecking on a dark body, as though the horse has been standing out in the snow collecting snow flakes over its back and sides. In most
cases the white spots increase in number and size as the horse ages. The
snowflakes can lie just across the
topline or may extend over the
whole body. Snowflake can also
occur in conjunction with other
patterns on the same horse.
Again, the snowflake will be heterozygous for Lp and probably
carries yet another version of
PATN which produces minimal
white patterning (Lp/lp PATN?/
?)
Frosting, whilst used by some to describe a horse with extensive roaning or
white hairs scattered over the back and hips where the roaning is not as extensive as
that seen in the Varnish, is considered by many others to be a term describing the
early stages of Varnish. If the concentration of white hairs is particularly heavy over
the rump the term ‘frosted blanket’ may be used.
Many thanks to some of our BCD owners for the use of these photos in this article.
It was great to be able to use photos of horses that we have registered with us to
illustrate some of the many and varied coat patterns that make up this most colourful
equine.
Page 24 - Kaleidoscope (Vol 55)
Courtesy of Sheila Archer from the Appaloosa Project.
This diagram shows the full spectrum of patterning observed in Appaloosas. The horses in the upper row have coat patterns
that indicate they are are heterozygous for LP, and therefore are not affected by CSNB. Note that they have white patterning with
moderate to plentiful dark spotting. The lower row of this diagram shows Appaloosas with coat patterns that indicate they are
homozygous for LP, and therefore affected by CSNB. Unlike the upper row horses, these have white patterning with few or no
dark spots.
More interesting stuff about Lp and PATN....
PATN can also be suppressed even when carried with LP and this appears to happen more commonly in mares for some
reason. Sometimes the only way of knowing that a mare carries PATN1 is when she produces a leopard or fewspot to a blanket
stallion. Owners have also noted that testosterone levels can influence pattern expression - again, not sure if this is proven or
just general observations.
Then there are the striped hooves. Not all appaloosas have striped hooves. Some have solid amber colored hooves. Usually,
but not always, the amber colored hooves are seen on horses that are homozygous for the leopard complex gene, or LpLp.
Agouti and Extension:
The presence, or absence, of agouti and extension, has also been documented to influence the expression of the ‘spotty’
genes in interesting ways. It has long been noted that red based horses (chestnut, palomino etc) on the whole have a greater
tendency towards white markings than black based horses and that brown (At) in particular has a tendency to ‘kill’ white
markings. This phenonemon has also been noted in horses carrying Lp and PATN where the expression of PATN appears,
according to information provided by owners and breeders, to be more extensive in red based horses. For instance, it has been
noted in horses which carry a similar Lp / PATN configuration that the black (and brown) ones have the least amount of
expression whilst the chestnuts have the most and that there also appears to be a dosage effect - a horse that is EE (homozygous
for black) has a more suppressed pattern than one which is Ee (carries a copy of both black and red) whilst ee (chestnut) has the
most expression.
Agouti (A), by contrast, seems to be an enhancing gene - a black based horse that carries agouti (so a bay) appears to have
more expression than a black based horse which doesn’t carry agouti (black) and a black based horse that carries 2 copies of
agouti has a bit more expression again than one which only has one copy.
All of which appears to indicate that black pigment suppresses white patterns (of all types) and that agouti allows more
expression again because it removes black pigment from the body. Hmmmmm, I see an article for a future edition of Kaleidoscope here.
I believe this tendency has also been noted by researchers involved in the Appaloosa Project, the research team responsible
for identifying and locating Lp in the DNA structure.
So....combine gender and colour and the comments about black mares that apparently carry Lp and PATN but don’t show
much evidence of either become understandable.
The effects of Sabino and Splash:
These are pattern enhancing genes. Sabino tends to make the pattern all lacy around the edges and makes the polka dots
smaller - it invades the coloured area and creates more white.
Splash makes the white stretch out. It’s a bit like a picture drawn on a balloon. As the balloon expands, the lines get further
apart.
So....how to get a horse with big polka dots. Horses with big polka dots seem to have no sabino, or one of the versions of
sabino that does not produce a lot of white. Therefore, horses with little to no face or leg white, and large patterns, are more
likely to have large polka dots.
Kaleidoscope (Vol 55 - Page 25)