A Small Problem: Dwarfism in
Dogs
Part
Three: Confusion Continues
(Warning: Pictures are slow to download. Please be patient.)
By
Fred Lanting
This is a follow-up
article to the one I wrote entitled “Osteochondrodysplasias”
in February of 2004. While that was a rather long piece, it still did not
address all that people want to know about the subject. Nor will this, but at
least we can look at some other aspects, including a little deeper delving into
the questions about the genetics of dwarfism.
There are
miniature and toy versions of “standard”-size breeds, but this is not the same
as dwarfism, the latter being the result of an abnormality rather than a
variation within normal limits in genes. People are always developing
miniaturized strains by selectively breeding small examples to each other, and
continuing to select until “regular” size individuals no longer appear. Some
years ago, the heiress to a margarine fortune started to develop miniature
Borzois. While some detractors accused her of using Whippets to jump-start the
reduction in size, it really doesn’t matter much. Livestock breeders know that
you can introduce a gene for some dominant characteristic such as color, but
then keep breeding the other structural phenotypes back into in the original
breed in such a way that the “new breed” (really a minor variation on the one
they started with) will look and perform no differently, except for that color.
Or whatever trait they want to introduce.
There is also
no reason to believe she did not simply choose the smallest Borzoi from her
extensive kennel and, in successive generations, bring down the size until the
partners would “breed true”, in regard to that characteristic while not losing
proportions or other qualities. Several years earlier, another woman developed
a strain of Boxers that matured at 12 pounds body weight by selective, not
cross-, breeding. While these examples never caught on, numerous such projects
have, to some extent: the Shar-Pei, Australian
Shepherd, Teacup Poodle, Bull Terrier, and numerous others. Miniatures do not
have enough genetic change to classify their genes or body phenotypes as
“abnormal” and, with the minor exception of a little difference in the head,
miniatures have the same proportions.
There is a type
of dwarfism that also produces proportional but suddenly smaller dogs. I say
“suddenly” because unlike the “breeding down” through many generations,
proportional dwarfs appear without successively smaller individuals in the line
of descent. So far, I have found the particular type that I am thinking of, in
only the German Shepherd Dog and in a breed with the GSD in its ancestry, the Karelian Bear Dog. Affected dogs are called Pituitary
Dwarfs because the immediate cause, or at least the noticeable defect, appears
in the hypophysis on the bottom of the brain. The
anterior lobe of this endocrine organ is rightly called the “master gland”
because of its governing or influential effect on other organs, specifically
the glands. Minor abnormalities in this gland are what create the body types of
Bulldogs, the acromegalic Saint Bernard, Dachshunds,
and endless other examples of a departure from the more “normal” or “ancestral”
types such as the GSD, sighthound, Pointer, etc.
Anatomic and functional abnormalities in different parts of the hypophysis make for the difference between the Boston Terrier and other breeds intentionally selected for their
abnormalities, for example.
Proportional
dwarfism in the GSD is called “pituitary dwarfism” because an old name for the hypophysis, or part of it, is “pituitary gland”. Since this
master gland controls much of the activity of other glands, it is not
surprising to see abnormalities in thyroid function, and thus the inability to
grow a normal coat. Most pituitary dwarfs look like Chinese Crested or other
“hairless” breeds although by carefully dosing with thyroid hormones (and
possibly the more expensive growth hormones), a normal
coat can be maintained. See my article in the December 1984 issue of Dog World,
which I may re-issue if there is enough interest. We can deduce that it is
caused by a defect in a different part of the pars distalis
of the hypophysis than different types of defects or
in different gland parts in other breeds. We can make such deductions because
such breed differences have been traced to such anatomic irregularities by Stockard and others at least as far back as the 1940s.
The
non-proportional canine dwarfs, like their human counterparts, result from
genetic defects that take root in other parts of this master gland, and
therefore other endocrine glands and organs. But there is much confusion,
disagreement, and lack of knowledge leading to frequently inadequate definitions.
In my other article, which you might call Part One of a trilogy, I mentioned
that various terms are used; I would like to here suggest that we settle on one
umbrella word to cover all or most others: either chondrodysplasia
or chondrodystrophy. The first simply means an
abnormal development or shape (-plasia) of cartilage
(chondr-). The latter is “translated” as poor (dys-) growth (troph-) of
cartilage. Either would be a less cumbersome term than I used as the title of
Part Two, osteochondrodysplasias, which includes the
“osteo-” simply to emphasize that the bones are also
abnormal. I think we do not need such a mouthful, and that readers will assume
the inclusion of shortened bones in the term “chondrodystrophy”.
A possible drawback to using chondrodysplasia is that
it might someday be confused with enchondromatosis, a
rare disease often involving tumors; these words are used interchangeably in
human medicine. On the other hand, chondrodystrophy
is sometimes used as part of a longer term for different disorders, also. Most of the time, though, it refers to a congenital defect in the
formation of bone from cartilage.
Achondroplasia is
one of those words that uses the prefix “a-” to denote
or connote an absence or deficiency of something. In this case, it means a lack
of (good) shape, growth, or form of the cartilage. Aplasia,
for example, means “lack of development”, as illustrated in my 2004 book by the
radiographic picture of an Airedale whose acetabulum
and top portion of the femur did not develop at all from cartilage. The achondroplastic limbs of the Dachshund means that these
extremities failed to elongate like the development in normal dogs. Achondroplasia of the skull is obvious in the Bulldog. In
either example, the word refers to a disordered chondrification
(and of course, later ossification) of the ends of bones. In most breeds, this
is most obvious in the long bones (limbs). It is simply arbitrary preference
that I use the words chondrodystrophy and chondrodysplasia more often.
But what about the genetics? To
even attempt to delve into the mysteries of inheritance of various forms of
dwarfism, one must be prepared to consider different genetic causes and
expressions in what, on first glance, is easy to assume are the same
conditions. Only by crossbreeding can we make better guesses. A couple of the
most active researchers into inheritance of traits and practitioners of
crossing breeds to get answers were Stockard in the
1920s to `40s, and Whitney in the `30s to `50s.
Basset
breeders know that achondroplasia is dominant in
their breed, and some think that this means the F1 progeny will always have the
same leg length as the Basset. But in crosses between Bassets and GSDs,
typically about half the legs (dogs) are intermediate in length, the other half
being normal (long, GSD-type) in length. The same when a Bassett-Bloodhound
with intermediate-length legs is crossed to a long-legged dog such as the
GSD or any other breed.
Cocker
Spaniels often have shorter-legged individuals, but the mutation to achondroplasia is not frequent, and is definitely
recessive. Other races breed true every time, such as Corgis. It appears that
“reverse mutation”, that is, a normal-leg-length offspring being produced by
two typical Corgis, just does not happen. Yet we know that we can suddenly find
Corgi-style legs in purebred pups of Cocker, German Shepherd,
and other breeds. Corgis (and dogs with this mutation suddenly appearing) may
have a slightly different genetic code and type of dwarfism than do Bassets and
Dachshunds. English Bulldogs seem to have a type of dwarfism more like the
Basset than the Corgi. The short legs of the Clumber Spaniel
or the Beagle are almost certainly not examples of dwarfism, as the shapes of
the joints and bones are more like those of the normal-length breeds. There is
still a great deal to be sorted out, when it comes to defining the genetic
differences in the dwarf dogs. Only when breeders are open and honest, and
share their experience and dogs with researchers, will we make progress in
unraveling the rest of this riddle.
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Miniature
Boxers. Proportional, but not dwarfs.
It is possible to miniaturize any breed by selection over many generations. |
Karelian Bear
Dog. Some cases of GSD pituitary
dwarfism have been found in this breed, but that is because there is some
German Shepherd ancestry in the lines. This is a normal dog. |
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Achondroplastic
dwarfism is a different type. It is quite rare in the GSD. These are
littermates. |
Many Cocker
Spaniels have short legs, but this one has true dwarf characteristics. Exact
type of dwarfism is unknown. |
Return Home
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Pituitary Dwarfism in the German
Shepherd Dog - Part One of a Three-part Set of Articles on Dwarfism |
Copyright 2001 Fred Lanting, Canine
Consulting. Mr.GSD@hiwaay.net. All rights reserved. Please view his site Real GSD.
NOTE: A well-respected AKC
and Schaferhund Verein judge,
Mr. Lanting has judged in more than a dozen
countries, including the prestigious FCI Asian Show hosted by Japan Kennel
Club, the Scottish Kennel Club, a Greyhound specialty in