Breeding – Dogs or
Pedigrees?
All dogs carry defective genes. These defective genes are usually “recessive” – that is, their
expression can be covered up by the presence of a normal gene for that
function. It is estimated that the
average dog carries 4 to 7 defective genes in it’s DNA. (The human estimate is
10 to 12). Since genes are always
carried in pairs, most of these abnormal genes are carried in a only single
dose, so that their presence is completely concealed by the other, normal
gene.
What is a gene?
A useful analogy is that a gene is like a set of instructions given to a
particular workman doing a small job on a very big construction site. Each workman gets two sets of plans. If one set is damaged, he still has one good
set, and the job can proceed. But if
both sets are damaged the job will not be finished, or it will be done
wrong. A gene is a large molecule, a long
double strand of DNA, composed of a backbone of two long sugar molecules linked
by pairs of smaller molecules called “bases” or “nucleotides”. It is the sequence of these nucleotides that
encodes the information contained in a gene.
How does a gene become defective? During the normal cell division, an
exact copy is made of each and every gene in the cell, and then it divides into
two daughter cells which are each an exact copy of the original cell. Defective genes are caused by a
“mutation”. If something happens to
disrupt the exact replication of the DNA during the cell division, a defective
gene results. Only a few changes in the
base sequence can render the information in that gene useless. The process of aging is undoubtedly the
effect of accumulated random defects of this sort, as are most types of cancer.
In the formation of egg and sperm, a special type of
division takes place. Instead of
replicating the genetic material, so that both the daughter cells have a full
complement of genes (two genes of each type), the genetic material is divided,
so that each reproductive cell has only one gene of each type. When sperm and egg finally meet, the full
complement of genes is restored, and a new individual, carrying half of its
mother’s genes and half of its father’s genes is created.
Selective breeding.
Nearly all breeding of domestic animals is selective as opposed to
random. Years ago, before the era of
scientific genetics, breeding was done more by phenotype than by pedigree. Race horses tended to be bred by the
stopwatch. That was where the money
was. Dairy cattle were bred by the
volume and quality of their milk, meat animals, by the speed of maturation and
ratio of feed to meat, and so on.
Later, it was recognized that breeding together closely related animals
tended to speed up the process of “fixing” the desired traits within a few
generations.
Breeding by pedigree is the type of selective
breeding most often practiced today. It
nearly always involves some degree of inbreeding. The logic is simple.
We know that an animal’s traits are genetically controlled. We can even calculate the percentage of a
particular animal’s genes residing in the cells of one of its descendants. When we mate closely related animals whose
family shows (has the phenotype of) the desired trait, we are reasonably sure
it will appear in the offspring. Some
breeders have carried this practice to remarkable extremes, failing to realize
there is a “catch” to the pedigree method.
What about those defective genes? The ones you can’t see because they are
“covered up” by intact ones. When we
breed closely related animals, (let us say because they have super rears), we
can see the desired trait. This trait
is genetically controlled, like all traits.
These two closely related animals share the gene for their super rears
as a result of their close genetic relationship. What we can’t see is the PRA gene or the kidney disease gene that
these two animals also share as a result of their close genetic
relationship. When we double up on the
good rears we are also doubling up on the particular hidden defect they share.
We can see the results of this type of concentration of
mutations in some human populations which have been relatively inbred by reason
of isolation due to status, geography, or religion. Some examples that come to mind are Tay-Sachs disease in eastern
European Jews, and hemophilia in some royal families.
Phenotype breeding has been neglected in recent
years. It has fallen into underserved
disrepute as the more popular inbreeding has produced faster and more dramatic
changes. I say undeservedly, because it
has much to recommend it, and avoids some of the serious pitfalls of
inbreeding.
Again, we look at phenotype of two relatively unrelated
animals. They both have good rears,
which we want. Why do they share this
trait? For the same reason that the two
related ones did: they both have the set of genes which produce good
rears. But what about the hidden, bad
genes? Since these animals could not
have been selected for unseen characteristics, (after all, if you can’t see it
you can’t consciously select for it), they probably do not share many of these
defective genes. To be sure, they still
carry their load of defects in their own private collections, but they most
likely each carry a different set. This
being the case, it is unlikely that any one of their offspring will inherit two
copies of the same defective gene. It
is very likely, however, that they will all have good rears.
Phenotype breeding is still selective breeding. We are selecting those animals which show
the desired traits, while minimizing the probability of doubling up on hidden,
undesired ones. Inbreeding and
linebreeding, one the other hand, selects for both the phenotypic and genotypic
traits, and dramatically increases the probability of producing animals
homozygous for defects.
The lesson in all this is that we should pay less
attention to pedigrees, particularly in terms of looking for similarities on
paper when we breed, and more attention to the dogs themselves. All too many breeders make their breeding
decisions on paper, and not in the flesh.
We need to consider the pedigrees as they relate to the qualities of the
parent animal – did his mom and dad have good rears – rather than to insist he
be related to our prospective brood bitch.
We can get the results we want by breeding unrelated “like to like”,
without the tragic by products of inbreeding.
A Glossary of Genetic Terms
Allels:
different versions of the same gene (found at the same locus but in homologous
chromosomes of in different individuals) that may produce different phenotypes.
Allele frequency: the fraction of all the alleles of a gene in a population that are
of one type.
Assortative mating:
a mating scheme that relies on the pairing of unrelated individuals
with similar phenotypes to obtain consistency of type and reinforce desirable
traits.
Codominant alleles:
two alleles that have different effects that are distinguishable in
a heterozygous individual (e.g. AB blood groups)
Cross-breeding: crossing
two different breeds
Dominant allele:
one that determines the phenotype even when there is only one copy (i.e.
in a heterozygous individual)
Drift: changes
in allele frequencies over time due to chance (as opposed to selection or
mutation)
Effective population size (Ne): the size of a hypothetical stable,
randomly-mating population that would have the same rate of gene loss or
increase in inbreeding as the real population (size N). As all finite populations are inbred to some
degree and generally do not choose mates at random, Ne is typically 1/10N or
less, particularly if fewer males breed than females.
Epistasis: used
to describe the situation where one gene’s expression prevents the expression
of another (e.g. you cannot determine whether an albino would have had black or
brown hair, though these two traits are controlled by separate genes.)
Fitness (relative):
The reproduction success of individuals of a particular genotype
relative to the most fit genotype.
Fixation: loss
of all alleles of a gene but one
Founder: an
individual drawn from a source population who contributes genetically to the
derived subpopulation.
Founder effect: changes
in allele frequencies that occur when a subpopulation is formed from a larger
one. Typically many rare and usually
undesirable alleles are excluded while a few carried by the founders get a big
boost in frequency.
Founder equivalents:
the number of hypothetical founders that would have the same diversity
as the descendant population. Generally
much smaller than the actual number due to unequal use and allele loss (gene
dropping).
Gene: that
portion of the genome that carries the information for a single protein. (In cases of proteins with multiple
subunits, there may be a gene for each.)
Gene dropping:
loss of alleles due to genetic drift
Genetic bottleneck:
when population numbers are temporarily reduced to a level insufficient
to maintain the diversity in the population
Genetic diversity:
usually expressed in terms of percentage of genes that are polymorphic
and/or are heterozygous.
Genome: the
total genetic makeup of an organism
Heritable:
passed on from parents to progeny through the chromosomes/DNA.
Heritability:
the fraction of the variability in a trait that is caused by genetic
differences
Heterozygous:
carrying two different alleles of a gene
Heterozygous advantage: a situation where the heterozygous genotype for a particular gene
shows the highest relative fitness
Heterozygous insufficiency: when the heterozygous genotype lacks sufficient gene product to
have the normal phenotype.
(Approximately equivalent to partial dominance.)
Heterosis: a
situation where crossing two inbred lines yields progeny that are more
healthy/vigorous than their parents.
(More commonly used in plant breeding.)
Homologous chromosomes: in higher plants and animals, chromosomes are found in nearly
identical “homologous” pairs, one coming from the sire and the other from the
dam. A dog has 39 pairs, or 78 in
total. Only one of each, chosen at
random, is passed on through eggs or sperm to the progeny.
Linebreeding:
a scheme that attempts to maintain a high contribution of one or two
ancestors through successive generations.
Often used by breeders for any inbreeding less intensive than between
first-degree relatives.
Linkage: a
measure of how frequently two genes found on the same chromosome remain
together during gamete (egg or sperm) formation.
Locus: the location
of a gene on a chromosome.
Map (aka linage map): a drawing showing the location of and relative distances between
genes on a chromosome.
Mean kinship (mk):
a measure of how related an individual is to the other members of a
population. Generally computed as the
average IC for the hypothetical progeny of the individual mated to all other
members of the population (both sexes).
A low average mk for a population indicates that most of the diversity
carried by the founders has been retained.
Monomorphic genes:
have only one common allele (rare alleles with frequencies of less
than 0.001% may still occur).
Mutation: a
change in the sequence of the base pairs in a DNA molecule.
Mutation rate:
the number of new mutations that occur per gene per gamete per
generation.
Outcrossing:
mating two individuals of the same breed that are sufficiently unrelated
that the IC of the progeny is lower than the average of the parents.
Polymorphic genes:
have 2 or more common alleles in the population
Recombination:
the reciprocal exchange of portions of two homologous chromosomes
(usually equivalent) during gamete formation.
Recombinant frequency (RF): how often two linked genes are separated by recombination,
generally expressed as a percentage of total progeny.
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