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Heritability
Fred Lanting
edited by John Cole
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Perhaps no term is more misunderstood
by dog fanciers when talking about inherited characteristics and environmental
effects than the concept of heritability. It is necessary for you to take
a little time and effort to get this idea fixed in your mind before applying
it to practical breeding programs. I will try to give the major ways of looking
at it as expressed by some of the leading authorities, in both veterinary
medicine and genetics. Try to differentiate between the similar terms of
heritability and inheritance. Think of HD as being 100% transmitted by genes,
while its expression (obvious or radiographic signs) is not as predictable
as that 100%. The first is how the trait is obtained, while the second is
how much of the trait is seen (obvious or radiographic signs). Pin this on
your wall:
INHERITANCE
= HOW; HERITABILITY = HOW MUCH.
Dr. Corley of the OFA describes heritability as a percentage measurement
of "how much of the phenotypic variation in a studied population is of genetic
origin." This means the differences caused by the genes, not the disease
itself. In the late 1950s and early '60s it was thought that heritability
of HD was around 0.6 (60%), but experiments in Sweden and elsewhere gave the
another heritability index value, that of 0.2 to 0.3 for the same breed.
Thus, a heritability index of 0.25 means that 25% of the phenotypic variance
(the expression of HD) is due to gene action. (This does not mean, however,
that the environment causes 75% of dysplasia in dogs.)
When Olmstead says, "70% of the development of hip dysplasia is influenced
by genetics" (personal correspondence, 1992), the key word is development.
Environment, then, would affect 30% of the development, including the severity,
age at which HD became apparent, and the specific nature of the lesions (osteophytes,
laxity, etc.). Environment would not have anything to do with the
actual existence (genotype) of the disorder. Therefore, to measure
the total phenotypic variance of HD, you must evaluate the pelvic radiographs
of a large population of dogs maintained in the same environment (identical
nutritional intake per pound of body weight, for example). Dr. Lust
of Cornell University maintains that the usefulness of heritability estimates
(indexes) is due to the fact that they are reliable predictors of future
progeny when certain phenotypes are combined in a breeding program.
Malcolm Willis, professor of genetics at the University of Newcastle-Upon-Tyne,
and one of Britain's leading HD/genetics authorities, reminds us that heritability,
strictly speaking, "relates to a specific character calculated on a specific
population of dogs at a specific point in time". You can more accurately
predict hip quality in your dogs' offspring, by decreasing the environmental
variation. Or you could outcross more often (always to dogs with good
joint backgrounds) to raise the genetic variance. This is like bringing
in more musicians to audition for spots in the orchestra. The more
you have from which to choose, the more you can cull of the less desirable
musicians and enhance the quality of your orchestra.
You can also positively direct heritability by fine tuning diagnostic
and rating systems. With the old BVA scheme, in which hips were classified
as normal, near normal, and dysplastic, HD heritability in the German Shepherd
Dog was said to be around 0.25, but when their much more detailed 0-to-106-point
scoring system was applied to the same dogs, heritability increased to nearly
0.4, which supposedly represents greater success in eliminating (or, at least,
reducing) the HD genotype from your line. If this is an accurate estimate,
then it means that the presence of good and bad genes have been more closely
identified by the newer British scheme than they had been by using the old
one, and the effects of environment are of lesser importance now that diagnosis
was supposedly more accurate. The OFA, using the AVMA protocol, is reported
to use a seven feature approach, also, but in practice two things are of
utmost importance in a diagnosis: laxity and any sign of remodelling or DJD.
This is not something they publicize to the breeder.
Continuing with the idea of directing heritability and changing the indexes,
remember that HD is polygenic. When many genes are involved, more accuracy
might be had in “looking” at them by more methods or aspects. Using an evaluation
scale with many levels for example, would allow the geneticist to determine
how much of the trait is under the control of genetics, and how much is under
the control of the environment. When you put all individuals into two or
three groups in order to evaluate them in terms of heritability, you do a
poor job of describing “reality” either statistically or verbally. Consider
an equation for heritability: h2 = s2A / (s2A +s2E). The better job
we do of estimating the environmental and genetic variances in the equation,
the better estimate we get of heritability, h2. Usually when a scale with
few categories is used to measure a trait that exhibits continuous variation,
genetic variance is underestimated and environmental variance is overestimated
(this has been going on in the field of HD for a long time), and h2 index
appears to be smaller than it should be.
Good advice has been given that breeders perhaps should not use the reported
and variable heritability figures either as a tool in breeding or as an excuse
to avoid using good genetic sense. We should not interpret heritability
to be anything more than an aid in predicting results from a breeding program.
Stick to the basics: HD (and other joint disease) is a genetic problem
and must be attacked by better selection of genes. You can raise the
heritability of your own lines by using the right mix of outcrossing and
linebreeding on dogs with the best phenotypes.
LINEBREEDING, OUTCROSSING, AND INBREEDING
Linebreeding (or its extreme application, inbreeding), while perpetuating
desirable characteristics also limits the gene pool. Another way of saying
this is that when you linebreed/inbreed, you end up with a smaller pedigree
(contains fewer unique individuals). This results in a reduction of additive
genetic variance because you have fewer genes to put together, and you have
many individuals with similar genotypes. That is why puppies from linebreeding
practices look so much more uniform than do pups resulting from outcrossing.
Since there is less variability and lower heritability in linebred dogs,
one can expect much less progress through phenotypic selection when the hips
of all the near ancestors have not been carefully "chosen." The typical
German Shepherd Dog is an example of extremely close linebreeding. In recent
decades this has been as true of the German dog as it has long been of the
phenotypically quite different AKC version. So, it is not surprising
that breeders who have not "culled the ancestors" and selected strongly for
good hips far back in the pedigree, will often find a "percentage plateau"
(a point at which no further progress can be made because of no more variability
in the line).
It is imperative that when you buy an outcrossed pup, you make sure that
the parents' hips are as good as possible. By obtaining all the information
you can on the parents' and their littermates and litters, especially the
dam's, the less likely you will be to encounter the "outcross surprise."
The October and December 1992 issues of Shiba Journal carried an excellent
piece on inbreeding by Susan Houser, an attorney with another degree in zoology
and direct work experience in genetics. She points out that inbreeding
(breeding of very close relatives) "brings about a decline in characters
concerned with fitness (viability, fecundity, and growth)”, quotes Darwin
in saying that prolonged inbreeding brings about "loss of size, constitutional
vigour, and fertility", and quotes Ehrlich and colleagues in saying, "a loss
of fitness referred to as inbreeding depression occurs . . . [when] . . .
inbreeding is imposed on populations that are usually outbreeding". Willis
also warns against unwise inbreeding. Inbreeding limits small populations
to small numbers of genes, and too often, many of these are the "wrong" genes
being paired because there is no dominant "good" allele there. While
inbreeding concentrates certain desired characteristics such as milk production,
rapid weight gain in pigs, more ears per cornstalk, grain yield per acre,
rear leg angulation, short legs, chiselled head features, etc., it also causes
the breed of animal to lose certain other genes through attrition or prevention
of being reintroduced to the pool. At least, the result is a loss of availability
of genes that are not easily recovered. The idea that genes are “lost” with
increasing homozygosity is referred to by population geneticists as “fixation”.
Inbreeding depression is the result of achieving homozygosity in recessive
genes. This could partly explain the reported rise in HD (9.7%) and
other defects among American German Shepherd Dogs while the rest of the world
population in that breed is steadily (but slowly) improving in hips, without
losing other historic qualities of type. Other breeds were reported
by the OFA to have increased in frequency of HD in the 1980s compared to
the 1970s: Great Dane 3.8%, Golden Retriever 1.3%, and Akita 1.1%.
The Labrador Retriever showed no change in HD, according to their statistics,
but had marginally more dogs graded Excellent.
In the light of this knowledge (or perhaps it is in spite of this knowledge
being available!) why do breeders use so much inbreeding, since there is
voluminous evidence that it badly affects health? Because they are focusing
on one or a few specific traits and, seeing some short-range success with
perhaps a litter of champions, continue to inbreed or heavily linebreed beyond
the invisible margin of safety. Then they wonder why their lines are
notorious for intussusception, pancreatic deficiency, and other organ problems
as well as shortened lifespans. The puppy buyers, rather than the breeders,
are too often forced to pay the price of inbreeding.
Fortunately, breeders can improve general health by selecting ancestors
for good joints without inbreeding. However, those who inbreed or
linebreed without regard to x-ray knowledge of their dogs' joints will undoubtedly
add dysplasias to the list of complaints of their doggy descendants.
PHENOTYPE SELECTION AND VARIABILITY IN REPORTING h2 INDEX
Heritabilitiesvary, both by the method of evaluating the trait and by
which particular traits are considered (e.g., HD's average heritability
may be different from heritability of shoulder angulation or temperament).
Heritability of HD varies also between breeds, so that the Samoyed, for example,
may have an estimated index variability of 0.8 and the German Shepherd Dog
may have one of 0.25 (we shall see later that this might be an inaccurate
and too low a figure) even if calculated by the same people. In Sweden, where
it is generally believed that heritability of HD is around 0.4 to 0.5 across
the board, you will still have differences between breeds. For example,
choose a German Shepherd Dog pup for breeding purposes from a group of "normal"
German Shepherd Dogs there and you might find, on average, more HD in its
pups (and perhaps the individual itself, when it grows up) than you would
if you did the same thing in Labradors.
Swenson explains this by the higher incidence of HD in their German Shepherd
Dog population, but I think it might have to do with differences in
heritability between breeds or colonies of dogs.
HD heritability can also be a function or property of a colony or population
of dogs selected over a period of time so that they have a different percentage
than other groups not so selectively bred. A conscientious breeder
who has been applying the best principles of HD prevention to his program
for generations may have a different index in his dogs than exists in the
breed at large. Heritability indices for HD in purebred dogs have been
estimated at levels as low as 0.2 and as high as 0.6 or even as high as 0.8
in Samoyeds, with most of the data supporting these numbers having been gathered
before much breeding progress in specific colonies. Reasons for why
heritability estimates vary have been more recently analysed and they are
as much dependent on the particular population (selected subgroup) as on the
environment, if not more.
Heritability differences between breeds can reflect the numbers of dogs
and families used by the amount of linebreeding and by differences in selection
practices. They may also reflect the number of grades and, hence, the
accuracy of the rating/scoring system used. Heritability differences
within a breed can be influenced by the same things. The reasons heritability
of HD in American guide dogs for the blind is so high (0.54) probably include
conscious pre-selection for better hips and possibly the use of other breeds
in addition to the "historic" German Shepherd Dog blind guide dog.
Traits that are high or moderately high in heritability (perhaps over
0.35, as an arbitrary level) respond comparatively rapidly to phenotype
selection (i.e., selection of breeding partners based on no radiographic
evidence of abnormalities in the joints). High heritability means
that environmental differences will have less effect on the expression of
bad genes, because there are fewer bad genes to be affected. In such
animals, phenotype is a better predictor of genotype than is the case in
those dogs with low heritability indexes. The polygenic trait of HD,
after generations of phenotypic breeding (i.e., exclusion of dysplastic dogs
from the program) begins to act more like a "simple" trait created by one
or two gene pairs.
For example, no matter what you do with environment, be it nutrition,
exercise, or anything else, you cannot change the phenotype of a black Labrador
Retriever to a yellow Labrador Retriever. Heritability of coat colour,
then, can be said to be 1.0 (100% controlled by the genes, zero by environment).
The yellow Lab doesn't have any genes for black and cannot produce black
puppies if bred to another yellow Lab. The goal of the breeder seriously
fighting HD is to develop a line of dogs with so few bad hip genes that in
a somewhat similar manner, it cannot produce differently than their own phenotypes.
At least, we should go as far in this direction as is feasible. If
you are dealing with a trait or a breed with low heritability (maybe 0.1 to
0.2 or so), selecting parental stock by phenotype (normal hips, in this context)
will yield slower progress in better hips in the first few generations.
However, if you keep at the selection process for normal hips, you will see
progress in your line. Stick with dogs that have been selected for
normal phenotype in your own and others' lines and you will raise the heritability
in your stock above that of the breed in general. Do you want to lower
the influence of environmental factors such as over nutrition on your production
of dogs? You can do so by selecting dogs that have hips that are relatively
unaffected by such factors, which is another way of saying you should choose
dogs with higher heritability.
On the other hand, the example used might not be the best, since it really
does not make a lot of sense to talk about heritability for a qualitative
trait such as coat colour. For one reason, you would have a hard time trying
to measure colour on an empirical scale in many breeds such as the Bloodhound,
Airedale, and German Shepherd, all of which are saddle marked dogs but with
differences in expression of the pattern. Such dogs are hard, if not impossible,
to refer to as “36% black” or “4% saddle”. Since there is no good numerical
value measured, you would not be able to compute the variances that form
the mathematical expression for heritability.
If all breeds are considered together, HD is a less heritable (and in
most cases more polygenic) trait than many other characteristics.
So, compared to those others, we would expect to see a less rapid change
in frequency when using strictly phenotypic breeding. This includes X-ray
pictures as well as regular visual signs and traits. When we speak of phenotype
in the context of a discussion on HD, we are referring to the evaluation
of the “standard” pelvic radiograph. If we say that a breed or population
within a breed has a heritability of 0.25, we are saying that the difference
in genes between two dogs is responsible for 25% of the x-ray picture differences
between them. That means that differences in environments (feeding practices,
road work, etc.) account for the other 75% of the phenotypic differences as
seen on the radiographs.
One reason for the fact that heritabilities can vary from one breed to
another is the history of intense linebreeding/inbreeding that is most
often used to found a breed. Look at the books on your own favourite breeds,
and you will find a very few individuals in all of the early pedigrees. A
natural consequence of such inbreeding is that there will be differences
between breeds in regard to genetic variation for the same trait.
In dairy cattle, heritabilities for traits such as volume of milk or level
of butterfat produced are quite similar, but that is because the major dairy
breeds, while heavily selected, have not undergone the same kind of pedigree
linebreeding that many dog breeds have. The Border Collie, as soon as it
was “accepted” by the AKC as a “show dog”, was doomed to eventual loss of
abilities, the same as happened to many a Spaniel, Setter, Shepherd, and “Non
Sporting” breed. Once breeding is based on pedigrees and a narrow slice of
the phenotype spectrum, instead of production records or verifiable working
ability, the gene pool becomes more limited, perhaps very much so.
The interplay of genetic and environmental components of the expression
of pelvic phenotype is complex and understandably misinterpreted by vets
and breeders alike. But the bottom line is still: environment may have
a great effect on the expression of genes an individual dog has inherited,
but no effect at all on the genes it will pass along to its progeny.
copyright
1999, Fred Lanting.
Reprinted with kind permission of Fred Lanting Author of The Total German Shepherd
Dog
This
is the expanded and enlarged second edition, a "must" for every true
GSD lover. It is an excellent alternative to the "genetic history" by
Willis, but less technical and therefore suitable for the novice, yet
very detailed to be indispensable for the reputable GSD breeder. Chapters
include: History and Origins, Modern Bloodlines, The Standard, Anatomy,
The German Shepherd in Motion, Shows, Showing, and Training, The Winners,
Nutrition and Feeding, General Care and Information, Health and First Aid,
Parasites and Immunity, Diseases and Disorders, The Geriatric German Shepherd,
Breeding, Basics of Genetics, Reproduction, Whelping, The First Three Weeks,
Four to Twelve Weeks, Trouble-shooting Guide
