2018 Miniature Horse World Magazine SUMMER Volume 34, Number 4 | Page 33

FISM
in
Miniatures
In 2013, the release of
an 83-page thesis written
by John Eberth shed
some light on dwarfism
for some. The full paper,
CHONDRODYSPLASIA-
LIKE DWARFISM IN THE
MINIATURE HORSE,
delves deep into the how
and why of dwarfism,
readily available online
at: https://uknowledge.
uky.edu/cgi/viewcontent.
cgi?article=1012
&context=gluck_etds
For many, the brief
synopsis below from the
University of Kentucky
is explanation enough,
reflecting the need
for more testing to be
accomplished with the
mares and stallions
who are producing the
Miniatures of the future.
The care of a dwarf can be very expensive.
A list of some (not all) health concerns for
dwarf Miniature horses include:
n Heart murmurs
n  Bad teeth, underbites, overbites,
mis-shapen mouths
n  Deformed hooves, twisted legs, loose
ligaments
n  Deformities of the face, muzzle and
nostrils causing difficulty breathing
n  Trouble eating and digesting feed due
to deformities both outwardly and
internally – a pot belly look can indicate
near normal sized digestive organs.
n  Hydrocephalis is a condition in
which there is an accumulation of
cerebrospinal fluid (CSF) within
the brain. Typically causes increased
pressure inside the skull.
n  Bad joints and ligaments, arthritis
University of Kentucky-
ACAN Dwarfism Mutations
in the Miniature Horse
Four mutations in the
ACAN gene have been
found in Miniature horses
associated with a dwarf
phenotype by John
Eberth, MS, working in
the lab of Dr. Ernie Bailey
at the Gluck Center. It is
important to test breeding
stock for these mutations
because at least one of

these mutations is lethal
in combination with any
of the other mutations,
causing early pregnancy
loss. Care must also be
taken in breeding two
horses together that
are carriers for Dwarf
mutations, as the presence
of two mutations in any
combination will lead to
early abortion or live foals
with a range of physical
ailments associated with
the dwarf phenotype.
Some of these physical
ailments seriously affect
the health of the horse
and include breathing
problems, malformed
mouths which lead to
eating difficulties, and
abnormal bone growth
leading to chronic
soundness issues.
Horses carrying only
one of the mutations are
normal in appearance,
exhibiting the desired
stature and proportions
the Miniature horse
should possess. There are
four identified mutations,
designated D1, D2, D3,
and D4. The normal copy
of the gene is designated
as N. The table below
summarizes the effects
of the various mutation
combinations.
Normal phenotype
The genotypes D3/D3
and D4/D4 have unknown
effects, as no samples with
these genotypes have
been found so far.
D1 and D2 were the
most common mutations
found in the pilot study. 
Since D1 is lethal in
combination with any of
the other mutations, it
is recommended that
D1 carriers (N/D1) only
be bred to horses that
do not carry any other
Dwarf mutation.
Currently, there are five
testable dwarf genes....
The 4 ACAN genes that
were studied and mapped
here and the Skeletal
Atavism, which is a
heritable defect leading
to abnormal development
of the skeleton structure
in the foals’ limbs. It is
caused by two different
mutations.  (Horses exhibit
a “dachshund dog” look, a
normal body with severely
shortened, sometimes
bowed legs) UC Davis at
California has the Skeletal
Atavism test now available
in the US (cost is $90).
Dwarf phenotype
Lethal (aborted/absorbed)
N/D1 D2/D2 D1/D1
N/D2 D2/D3 D1/D2
N/D3 D2/D4 D1/D3
N/D4 D3/D4 D1/D4
Continued on page 33.
S U M M E R 2018
Miniature Horse World 31