IT'S ALL IN THE GENES
As dog breeders, we engage in genetic
"experiments" each time we plan a mating. The type of mating
selected should coincide with your goals. To some breeders,
determining which traits will appear in the offspring of a mating is
like rolling the dice - a combination of luck and chance. For
others, producing certain traits involves more skill than luck - the
result of careful study and planning. As breeders, we must
understand how we manipulate genes within our breeding stock to
produce the kinds of dogs we want. We have to first understand dogs
as a species, then dogs as genetic individuals.
The species, Canis familiaris, includes all
breeds of the domestic dog. Although we can argue that there is
little similarity between a Chihuahua and a Saint Bernard, or that
established breeds are separate entities among themselves, they all
are genetically the same species. While a mating within a breed may
be considered out bred, it still must be viewed as part of the whole
genetic picture: a mating within an isolated, closely related,
interbred population. Each breed was developed by close breeding and
inbreeding among a small group of founding canine ancestors, either
through a long period of genetic selection or by intensely
inbreeding a smaller number of generations. The process established
the breed's characteristics and made the dogs in it breed true.
When evaluating your breeding program, remember
that most traits you're seeking cannot be changed, fixed or created
in a single generation. The more information you can obtain on how
certain traits have been transmitted by your dog's ancestors, the
better you can prioritize your breeding goals. Tens of thousands of
genes interact to produce a single dog. All genes are inherited in
pairs, one pair from the father and one from the mother. If the pair
of inherited genes from both parents is identical, the pair is
called homozygous. If the genes in the pair are not alike, the pair
is called heterozygous. Fortunately, the gene pairs that make a dog
a dog and not a cat are always homozygous. Similarly, the gene pairs
that make a certain breed always breed true are also homozygous. .
Therefore, a large proportion of homozygous non-variable pairs -
those that give a breed its specific standard - exist within each
breed. It is the variable gene pairs, like those that control color,
size and angulation, that produce variations within a breed.
BREEDING BY PEDIGREE
Out breeding brings together two dogs less
related than the average for the breed. This promotes more
heterozygosity, and gene diversity within each dog by matching pairs
of unrelated genes from different ancestors. Out breeding can also
mask the expression of recessive genes, and allow their propagation
in the carrier state.
Most out breeding tends to produce more variation
within a litter. An exception would be if the parents are so
dissimilar that they create a uniformity of heterozygosity. This is
what usually occurs in a mismating between two breeds. The resultant
litter tends to be uniform, but demonstrates "half-way points"
between the dissimilar traits of the parents. Such litters may be
phenotypically uniform, but will rarely breed true due to the mix of
dissimilar genes.
A reason to outbreed would be to bring in new
traits that your breeding stock does not possess. While the parents
may be genetically dissimilar, you should choose a mate that
corrects your dog's faults but phenotypically complements your dog's
good traits.
It is not unusual to produce an excellent quality
dog from an out bred litter. The abundance of genetic variability
can place all the right pieces in one individual. Many top-winning
show dogs are out bred. Consequently, however, they may have low
inbreeding coefficients and may lack the ability to uniformly pass
on their good traits to their offspring. After an out breeding,
breeders may want to breed back to dogs related to their original
stock, to increase homozygosity and attempt to solidify newly
acquired traits.
Line breeding attempts to concentrate the genes
of a specific ancestor or ancestors through their appearance
multiple times in a pedigree. The ancestor should appear behind more
than one offspring. If an ancestor always appears behind the same
offspring, you are only line breeding on the approximately 50
percent of the genes passed to the offspring and not the ancestor
itself.
It is better for line bred ancestors to appear on
both the sire's and the dam's sides of the pedigree. That way their
genes have a better chance of pairing back up in the resultant pups.
Genes from common ancestors have a greater chance of expression when
paired with each other than when paired with genes from other
individuals, which may mask or alter their effects.
A line breeding may produce a puppy with
magnificent qualities, but if those qualities are not present in any
of the ancestors the pup has been line bred on, it may not breed
true. Therefore, careful selection of mates is important, but
careful selection of puppies from the resultant litter is also
important to fulfill your genetic goals. Without this, you are
reducing your chances of concentrating the genes of the line bred
ancestor.
Increasing an individual's homozygosity through
line breeding may not, however, reproduce an out bred ancestor. If
an ancestor is out bred and generally heterozygous (Aa), increasing
homozygosity will produce more AA and aa. The way to reproduce an
out bred ancestor is to mate two individuals that mimic the
appearance and pedigree of the ancestor's parents.
Inbreeding significantly increases homozygosity,
and therefore uniformity in litters. Inbreeding can increase the
expression of both beneficial and detrimental recessive genes
through pairing up. If a recessive gene (a) is rare in the
population, it will almost always be masked by a dominant gene (A).
Through inbreeding, a rare recessive gene (a) can be passed from a
heterozygous (Aa) common ancestor through both the sire and dam,
creating a homozygous recessive (aa) offspring. Inbreeding does not
create undesirable genes, it simply increases the expression of
those that are already present in a heterozygous state.
Inbreeding can exacerbate a tendency toward
disorders controlled by multiple genes, such as hip dysplasia and
congenital heart anomalies. Unless you have prior knowledge of what
milder linebreedings on the common ancestors have produced,
inbreeding may expose your puppies (and puppy buyers) to
extraordinary risk of genetic defects. Research has shown that
inbreeding depression, or diminished health and viability through
inbreeding is directly related to the amount of detrimental
recessive genes present. Some lines thrive with inbreeding, and some
do not.
PEDIGREE ANALYSIS
Geneticists' and breeders' definitions of
inbreeding vary. A geneticist views inbreeding as a measurable
number that goes up whenever there is a common ancestor between the
sire's and dam's sides of the pedigree; a breeder considers
inbreeding to be close inbreeding, such as father-to-daughter or
brother-to-sister mating. A common ancestor, even in the eighth
generation, will increase the measurable amount of inbreeding in the
pedigree.
The Inbreeding Coefficient (or Wright's
coefficient) is an estimate of the percentage of all the variable
gene pairs that are homozygous due to inheritance from common
ancestors. It is also the average chance that any single gene pair
is homozygous due to inheritance from a common ancestor. In order to
determine whether a particular mating is an out breeding or
inbreeding relative to your breed, you must determine the breed's
average inbreeding coefficient. The average inbreeding coefficient
of a breed will vary depending on the breed's popularity or the age
of its breeding population. A mating with an inbreeding coefficient
of 14 percent based on a ten generation pedigree, would be
considered moderate inbreeding for a Labrador Retriever (a popular
breed with a low average inbreeding coefficient), but would be
considered out bred for an Irish Water Spaniel (a rare breed with a
higher average inbreeding coefficient).
For the calculated inbreeding coefficient of a
pedigree to be accurate, it must be based on several generations.
Inbreeding in the fifth and later generations (background
inbreeding) often has a profound effect on the genetic makeup of the
offspring represented by the pedigree. In studies conducted on dog
breeds, the difference in inbreeding coefficients based on four
versus eight generation pedigrees varied immensely. A four
generation pedigree containing 28 unique ancestors for 30 positions
in the pedigree could generate a low inbreeding coefficient, while
eight generations of the same pedigree, which contained 212 unique
ancestors out of 510 possible positions, had a considerably higher
inbreeding coefficient. What seemed like an out bred mix of genes in
a couple of generations, appeared as a line bred concentration of
genes from influential ancestors in extended generations.
The process of calculating coefficients is too
complex to present here. Several books that include how to compute
coefficients are indicated at the end of this article; some
computerized canine pedigree programs also compute coefficients. The
analyses in this article were performed using CompuPed, by RCI
Software.
[RCI Note: CompuPed computes Wright's Inbreeding
Coefficient faster and more accurately than any other PC program
available. ]
To visualize some of these concepts, please refer
to the above pedigree. Line bred ancestors in this pedigree are in
color, to help visualize their contribution. The paternal grandsire,
CH Loch Adair Foxfire, and the maternal granddam, CH Loch Adair
Firefly WD, are full siblings, making this a first-cousin mating.
The inbreeding coefficient for a first cousin mating is 6.25%, which
is considered a mild level of inbreeding. Lists of inbreeding
coefficients based on different types of matings are shown in the
table below.
In Bilye's pedigree, an inbreeding coefficient
based on four generations computes to 7.81%. This is not
significantly different from the estimate based on the first-cousin
mating alone. Inbreeding coefficients based on increasing numbers of
generations are as follows: five generations, 13.34%; six
generations, 18.19%; seven generations, 22.78%; eight generations,
24.01%; ten generations, 28.63%; and twelve generations, 30.81%. The
inbreeding coefficient of 30.81 percent is more than what you would
find in a parent-to-offspring mating (25%). As you can see, the
background inbreeding has far more influence on the total inbreeding
coefficient than the first-cousin mating, which only appears to be
its strongest influence.
Knowledge of the degree of inbreeding in a
pedigree does not necessarily help you unless you know whose genes
are being concentrated. The percent blood coefficient measures the
relatedness between an ancestor and the individual represented by
the pedigree. It estimates the probable percentage of genes passed
down from a common ancestor. We know that a parent passes on an
average of 50% of its genes, while a grandparent passes on 25%, a
great-grandparent 12.5%, and so on. For every time the ancestor
appears in the pedigree, its percentage of passed-on genes can be
added up and its "percentage of blood" estimated.
In many breeds, an influential individual may not
appear until later generations, but then will appear so many times
that it necessarily contributes a large proportion of genes to the
pedigree. This can occur in breeds, due to either prolific ancestors
(usually stud dogs), or with a small population of dogs originating
the breed. Based on a twenty-five generation pedigree of Bilye,
there are only 852 unique ancestors who appear a total of over
twenty-million times.
Pedigree Analysis of Laurel Hill Braxfield Bilye
(computed to 25 generations)
| |
|
1st Generation
|
|
Line bred Ancestors
|
Percentage of blood
|
Appearance in pedigree
|
# times in pedigree
|
| |
|
|
|
| CH Afternod Drambuie |
33.20% |
6 |
33 |
| CH Afternod Sue |
27.05% |
7 |
61 |
| CH Afternod Callant |
26.56% |
5 |
13 |
| |
|
|
|
| "Grand-Parents" |
25.00% |
2 |
1 |
| CH Sutherland Gallant |
25.00% |
3 |
2 |
| CH Sutherland MacDuff |
25.00% |
3 |
3 |
| CH Sutherland Lass of Shambray |
25.00% |
3 |
2 |
| CH Wilson's Corrie, CD |
22.30% |
7 |
200 |
| CH Afternod Buchanon |
20.22% |
7 |
48 |
| Loch Adair Diana of Redchic |
17.97% |
5 |
12 |
| CH EEG's Scotia Nodrog Rettes |
17.76% |
8 |
181 |
| Afternod Ember of Gordon Hill |
17.14% |
8 |
76 |
| CH Afternod Hickory |
16.21% |
6 |
27 |
| CH Black Rogue of Serlway |
15.72% |
9 |
480 |
| CH Afternod Woodbine |
14.45% |
6 |
15 |
| CH Fast's Falcon of Windy Hill |
13.82% |
8 |
66 |
| Afternod Fidemac |
13.67% |
5 |
7 |
| CH Page's MacDonegal II |
13.43% |
7 |
56 |
| Afternod Hedera |
13.38% |
7 |
56 |
| CH Downside Bonnie of Serlway |
12.90% |
10 |
708 |
| Peter of Crombie |
12.76% |
11 |
3,887 |
| |
|
|
|
| "Great-Grand-Parents" |
12.50% |
3 |
1 |
| CH Afternod Amber |
12.50% |
5 |
5 |
| Ben of Crombie |
11.83% |
11 |
7,584 |
| Stylish William |
11.18% |
13 |
23,764 |
| Stylish Billie |
11.08% |
14 |
70,542 |
| Stylish Ranger |
10.80% |
15 |
297,331 |
| CH Afternod Kate |
10.74% |
6 |
17 |
| Heather Grouse |
10.61% |
16 |
1,129,656 |
| Afternod Hedemac |
10.45% |
7 |
28 |
The above analysis shows the ancestral
contribution of the line bred ancestors in Bilye's pedigree. Those
dogs in color were present in the five-generation pedigree. CH
Afternod Drambuie has the highest genetic contribution of all of the
line bred ancestors. He appears 33 times between the sixth and
eighth generations. One appearance in the sixth generation
contributes 1.56% of the genes to the pedigree. His total
contribution is 33.2% of Bilye's genes, second only to the parents.
Therefore, in this pedigree, the most influential ancestor doesn't
even appear in the five-generation pedigree. His dam, CH Afternod
Sue, appears 61 times between the seventh and tenth generations, and
contributes more genes to the pedigree than a grandparent.
Foundation dogs that formed the Gordon Setter
breed also play a great role in the genetic makeup of today’s dogs.
Heather Grouse appears over one million times between the sixteenth
and twenty-fifth generations, and almost doubles those appearances
beyond the twenty-fifth generation. He contributes over ten percent
of the genes to Bilye’s pedigree. This example shows that the depth
of the pedigree is very important in estimating the genetic makeup
of an individual. Any detrimental recessive genes carried by Heather
Grouse or other founding dogs, would be expected to be widespread in
the breed.
BREEDING BY APPEARANCE
Many breeders plan matings solely on the
appearance of a dog and not on its pedigree or the relatedness of
the prospective parents. This is called assortative mating. Breeders
use positive assortative matings (like-to-like) to solidify traits,
and negative assortative matings (like-to-unlike) when they wish to
correct traits or bring in traits their breeding stock may lack.
Some individuals may share desirable
characteristics, but they inherit them differently. This is
especially true of polygenic traits, such as ear set, bite, or
length of forearm. Breeding two phenotypically similar but
genotypically unrelated dogs together would not necessarily
reproduce these traits. Conversely, each individual with the same
pedigree will not necessarily look or breed alike.
Breedings should not be planned solely on the
basis of the pedigree or appearance alone. Matings should be based
on a combination of appearance and ancestry. If you are trying to
solidify a certain trait - like topline - and it is one you can
observe in the parents and the line bred ancestors of two related
dogs, then you can be more confident that you will attain your goal.
GENETIC DIVERSITY
Some breed clubs advocate codes of ethics that
discourage line breeding or inbreeding, as an attempt to increase
breed genetic diversity. This position is based on a false premise.
Inbreeding or line breeding does not cause the loss of genes from a
breed gene pool. It occurs through selection; the use and non-use of
offspring. If some breeders line breed to certain dogs that they
favor, and others line breed to other dogs that they favor, then
breed-wide genetic diversity is maintained.
In a theoretical mating with four offspring, we
are dealing with four gene pairs. The sire is homozygous at 50% of
his gene pairs (two out of four), while the dam is homozygous at 75%
of her gene pairs. It is reasonable to assume that she is more
inbred than the sire.
A basic tenet of population genetics is that gene
frequencies do not change from the parental generation to the
offspring. This will occur regardless of the homozygosity or
heterozygosity of the parents, or whether the mating is an out
breeding, line breeding, or inbreeding. This is the nature of
genetic recombination.
There is a lack of gene diversity at the first
(olive) gene pair, so that only one type of gene combination can be
produced: homozygous olive. As the sire is homozygous lime at the
third gene pair, and the dam is homozygous blue, all offspring will
be heterozygous at the third gene pair. Depending on the dominant or
recessive nature of the blue or lime genes, all offspring will
appear the same for this trait due to a uniformity of
heterozygosity.
If offspring D is used as a prolific breeder, and
none of the other offspring are bred to a great extent, gene
frequencies in the breed will change. As dog D lacks the orange gene
in the second pair and the purple gene in the fourth pair, the
frequencies of these genes will diminish in the breed. They will be
replaced by higher frequencies of the red and pink genes. This
shifts the gene pool, and the breed’s genetic diversity. Of course,
dogs have more than four gene pairs, and the overuse of dog D to the
exception of others can affect the gene frequency of thousands of
genes. Again, it is selection (for example of dog D to the exception
of others), and not the types of matings he is involved in that
alters gene frequencies.
Breeders should select the best individuals from
all kennel lines, so as to not create new genetic bottlenecks. There
is a tendency for many breeders to breed to a male; who produced no
epileptics in matings to several epileptic dams, to an OFA excellent
stud, or to the top winning dog in the show ring. Regardless of the
popularity of the breed, if everyone is breeding to a single stud
dog, (the popular sire syndrome) the gene pool will drift in that
dog’s direction and there will be a loss of genetic diversity. Too
much breeding to one dog will give the gene pool an extraordinary
dose of his genes, and also whatever detrimental recessives he may
carry, to be uncovered in later generations. This can cause future
breed related genetic disease through the founders effect.
Dogs who are poor examples of the breed should
not be used simply to maintain diversity. Related dogs with
desirable qualities will maintain diversity, and improve the breed.
Breeders should concentrate on selecting toward a breed standard,
based on the ideal temperament, performance, and conformation, and
should select against the significant breed related health issues.
Using progeny and sib-based information to select against both
polygenic disorders and those without a known mode of inheritance
will allow greater control.
Rare breeds with small gene pools have concerns
about genetic diversity. What constitutes acceptable diversity
versus too restricted diversity? The problems with genetic diversity
in purebred populations concern the fixing of deleterious recessive
genes, which when homozygous cause impaired health. Lethal
recessives place a drain on the gene pool either prenatally, or
before reproductive age. They can manifest themselves through
smaller litter size, or neonatal death. Other deleterious recessives
cause disease, while not affecting reproduction.
Problems with a lack of genetic diversity arise
at the gene locus level. There is no specific level or percentage of
inbreeding that causes impaired health or vigor. It has been shown
that some inbred strains of animals thrive generation after
generation, while others fail to thrive. If there is no diversity
(non-variable gene pairs for a breed) but the homozygote is not
detrimental, there is no effect on breed health. The characteristics
that make a breed reproduce true to its standard are based on
non-variable gene pairs. A genetic health problem arises for a breed
when a detrimental allele increases in frequency and homozygosity.
GENETIC CONSERVATION
The perceived problem of a limited gene pool has
caused some breeds to advocate out breeding of all dogs. Studies in
genetic conservation and rare breeds have shown that this practice
actually contributes to the loss of genetic diversity. By uniformly
crossing all "lines" in a breed, you eliminate the differences
between them, and therefore the diversity between individuals. This
practice in livestock breeding has significantly reduced diversity,
and caused the loss of unique rare breeds. The process of
maintaining healthy "lines" or families of dogs, with many breeders
crossing between lines and breeding back as they see fit maintains
diversity in the gene pool. It is the varied opinion of breeders as
to what constitutes the ideal dog, and their selection of breeding
stock that maintains breed diversity.
The Doberman Pincher breed is large, and
genetically diverse. The breed has a problem with von Willebrands
disease, an autosomal recessive bleeding disorder. Some researchers
estimate that up to 60% of the breed may be homozygous recessive for
the defective gene, and the majority of the remaining dogs are
heterozygous. Therefore, there is diminished genetic diversity in
this breed at the vonWillibrands locus. A genetic test and screening
program now exists for Doberman Pincher breeders. They can identify
carrier and affected dogs, and decrease the defective gene frequency
through selection of normal testing offspring for breeding. By not
just eliminating carriers, but replacing them with normal testing
offspring, genetic diversity will be conserved.
Dalmatians have a high frequency defective
autosomal recessive gene controlling purine metabolism. Homozygous
recessive individuals can have urinary problems due to urate bladder
stones and crystals, and an associated skin condition (Dalmatian
Bronzing Syndrome). At one time, the breed and the AKC approved a
crossbreeding program to a few Pointers, to bring normal purine
metabolism genes into the gene pool. The program was abandoned for
several reasons, but it was accepted that the number of individual
Dalmatians with two normal purine metabolism genes far exceeded the
few Pointers that were being used in the program. The impact of
other Pointer genes foreign to the Dalmatian gene pool could have
had a greater detrimental effect than the few normal purine
metabolism genes being imported through the program.
PUTTING IT ALL TOGETHER
Decisions to line breed, inbreed or outbreed
should be made based on the knowledge of an individual dog's traits
and those of its ancestors. Inbreeding will quickly identify the
good and bad recessive genes the parents share in the offspring.
Unless you have prior knowledge of what the pups of milder
linebreedings on the common ancestors were like, you may be exposing
your puppies (and puppy buyers) to extraordinary risk of genetic
defects. In your matings, the inbreeding coefficient should only
increase because you are specifically line breeding (increasing the
percentage of blood) to selected ancestors.
Don't set too many goals in each generation, or
your selective pressure for each goal will necessarily become
weaker. Genetically complex or dominant traits should be addressed
early in a long-range breeding plan, as they may take several
generations to fix. Traits with major dominant genes become fixed
more slowly, as the heterozygous (Aa) individuals in a breed will
not be readily differentiated from the homozygous-dominant (AA)
individuals. Desirable recessive traits can be fixed in one
generation because individuals that show such characteristics are
homozygous for the recessive genes. Dogs that breed true for
numerous matings and generations should be preferentially selected
for breeding stock. This prepotency is due to homozygosity of
dominant (AA) and recessive (aa) genes.
If you line breed and are not happy with what you
have produced, breeding to a less related line immediately creates
an outbred line and brings in new traits. Repeated outbreeding to
attempt to dilute detrimental recessive genes is not a desirable
method of genetic disease control. Recessive genes cannot be
diluted; they are either present or not. Outbreeding carriers
multiplies and further spreads the defective gene(s) in the gene
pool. If a dog is a known carrier or has high carrier risk through
pedigree analysis, it can be retired from breeding, and replaced
with one or two quality offspring. Those offspring should be bred,
and replaced with quality offspring of their own, with the hope of
losing the defective gene.
Trying to develop your breeding program
scientifically can be an arduous, but rewarding, endeavor. By taking
the time to understand the types of breeding schemes available, you
can concentrate on your goals towards producing a better dog.
Further Reading:
If you are interested in learning more about these subjects,
consult the following books:
-
Abnormalities of Companion Animals: Analysis of
Heritability
C.W. Foley, J.F. Lasley, and G.D. Osweiler, Iowa State University
Press, Ames, Iowa. 1979.
-
Genetics for Dog Breeders
F.B. Hutt, W.H. Freeman Co, San Francisco, California. 1979.
-
Veterinary Genetics
F. W. Nicholas, Clarendon Press, Oxford England. 1987.
-
Genetics for Dog Breeders
R. Robinson, Pergamon Press, Oxford England. 1990.
-
Genetics of the Dog (equally applicable to cats &
other animals)
M.B. Willis, Howell Book House, New York, New York. 1989.