Genetic Diversity - the Dark Side of Inbreeding
Catherine Marley
If you can imagine building a
house, there are numerous subcontractors working on it, the framer, the
electrician, the plumber, the bricklayer etc. Now each worker gets two sets of
plans for his part of the job. Of course there are a number of other sets of
plans or blueprints for this kind of job back at the main office, but for the
job in this house, the worker only gets two of the many possible plans.
A gene is like a long
blueprint given to the subcontractors in the body. If there is a
mistake in the blueprint, the worker either does the job incorrectly or he
doesn't finish the job. All the possible plans (or genes) that one might
use for a particular job are termed alleles. In this case, the worker
gets two sets of plans (genes) of the many possible ones (alleles), and he puts
one set in each hip pocket. The pocket is the analagous to the "gene
locus". The locus is an actual location on each of a pair of
chromosomes in the cell. Each cell has multiple pairs of chromosomes, -
the number is species specific. Each chromosome contains many loci for all the
jobs that have to be done in the body. There is one allele, specific for
each job, residing at the identical locus on each of a pair of chromosomes,
- just like the two sets of blueprints in the worker's hip pockets. The animal
got one set of those paired chromosomes from each of it's parents. In the
carrier state of a recessive genetic mutation, the animal has gotten one good
copy of the gene in question, and one defective copy. In this case, one good
copy is sufficient for the work to proceed and the animal to be healthy.
Genetic diversity in a
population means that the population contains most of the possible alleles
(alternate sets of plans) for a particular gene locus rather evenly distributed
throughout the population. (Of course an individual animal can only have two of
those alleles, for every gene locus, in his or her private collection.) This is
where you start getting into trouble in an isolated population, such as in
quarantine countries, or in very small countries. A very popular stud arrives
in the country. Soon every bitch in the population is bred to him. Suddenly
every puppy in the country has one or the other of his alleles for every gene
locus. By breeding every bitch to the one male, you have selected
AGAINST all the other alleles in the adult male population. These
alleles will disappear unless they are handed down to offspring. This is
genetic death. (death before having a chance to reproduce). Genetic diversity
has been lost.
Let us suppose that there were
200 males, and 200 females in our hypothetical breeding population. But all 200
bitches were bred to Mr. Wonderful. Now, instead of having 200 different
paternal gene collections represented in those 200 litters, we have only one.
Now someone says, "We got such great type from this dog, let's breed his daughters
to him as well." Now you are throwing away another whole set of
alleles.
Now suppose this dog carries a
hidden defect. In the first 200 litters sired by Mr. Wonderful, one half of the
pups are carriers of that same gene. In selecting for the gene set that
this dog carries, you have selected FOR the defective gene, and AGAINST a
"good" one which may have been in the population before Mr.
Wonderful came along.
Lets say that by now, (which is
usually the case in popular sire effect), the overall carrier rate for the
bad gene is 50%. This means that every other animal in the population
carries at least one copy of Mr. Wonderful's defective gene. The chances are 1/2
that any dog carries it and 1/2 that any bitch carries it. The chances are
1/2 x 1/2 or one in 4 that two carriers will be mated, even if we bred them
randomly. But by now Mr. Wonderful’s "look" has become a
showring necessity. In all likelihood, we will be selecting, for our breeding,
that dog and bitch who look most like Mr. Wonderful. The very plain pair have
been neutered and placed a pets.
Again we have selected for the
genes of Mr. Wonderful and against the other genes in the original
population. But the pair resembling their famous sire are the very ones
most likely to be carrying the heaviest helping of Mr. Wonderful genes, both
good and bad. So the chances are that more than 3 out of four litters will
produce carriers, and one out of 4 litters will now have affected puppies.
In each of these affected litters, 3 out of 4 pups will carry the defective
gene. Only one in four pups in each of these litters will be "clean".
What is the solution?
The most obvious response is "don't breed any dogs carrying that
gene". Now suppose we decide we will not permit breeding of
affected dogs or carriers. Abruptly, we eliminate from our breeding the the 50%
of the population which carry the defective gene. Which dogs are most likely to
be carriers? The ones that look like Mr. Wonderful, of course. What is now left
in this genetically decimated population? Precious little, perhaps not enough
to go on with. There may be genetic problems that have surfaced because of the
lack of "good" genes in the population, neonatal losses, failure to
thrive, allergies, etc.
What happens, on the other hand,
if we outcross to a line which has never had this problem? Suppose
we assemble a number of unrelated dogs, - lets call them all "Mr. Clean"
- and breed each of our bitches to a different Mr. Clean. Again 1/2 of the
bitches which are bred to our Mr. Cleans, are carriers. This means that they
have one "bad" copy of the gene, and one "good" copy. Since
the sires have only good copies of the gene, half of the pups from the
carrier mothers will be carriers, but NONE will be affected. In a
late onset disease, identifying affected animals before they are bred will be a
problem, but testing for the disease before breeding will reduce the number of
litters an affected animal is likely to have. By using mainly outcrosses,
even the accidental use of affected dogs will not increase the prevalence of
affected offspring.
What about other problems? If
this group of sires carry undesirable traits, they will each have a different
collection, - alleles different from the inbred population and from each other.
The likelihood of two "bad" copies of any gene getting together is
thereby markedly reduced. And this infusion of new genes has introduced new
genetic diversity into the impoverished line.
Put these two approaches
together. Let's say we do not intentionally breed any animals who carry two
copies of the defect, and we only use known carriers when there is a
good reason to do so, and then ONLY to unrelated animals. This approach
allows the gradual removal of the defective gene from the population, and the
preservation of most of the good qualities of good old Mr. Wonderful. It
restores a healthy degree of genetic vigor to the population, and lessens the
likelihood that other "bad" problems will manifest themselves.
This is what the "genetic
diversity" approach to breeding is all about. The "wholesale genetic
slaughter" method may be appropriate in a genetically diverse population
with only an occasional individual case of the disease in question. (Of course
you rarely have this kind of problem in a genetically diverse population.) You
get into the heavily "loaded" state only by inbreeding. Using the
"wholesale genetic slaughter" method, even if it were practically
possible, will not cure the problem in this inbred population, because the
real culprit is not the defective gene, but the inbreeding. Besides you
have to wonder what you will have left when you are finished. The dogs are
still inbred, with all the problems that go along with inbreeding. In
fact, they are twice as inbred as when you started, because you threw
away half the genes. You won't have much of the genetic disease you select
against, but maybe you won't have dogs either.
A reasonable course of action
in any genetic disease demands that attention be paid both to removing the gene
where possible without seriously degrading the genetic viability of the
population, and taking steps to provide increased genetic diversity if it is
needed.
Email Catherine
Marley
This article was first published on the Kai-La-Sha website.
Reproduced here by permission.