ELIMINATING MUTATION:
THE
IMPOSSIBLE DREAM
by John Armstrong
Though it is not practical to eliminate all deleterious mutation,
the incidence of affected individuals may be significantly reduced through a
combination of intelligent breeding practice and the development of DNA tests.
Why do we have
mutations?
Mutations are changes in an organism's DNA that potentially affect
the correct functioning of genes. They occur naturally due to replication
errors, mispairing of homologous chromosomes, or
through unavoidable exposure to natural radiation (e.g., cosmic rays). Mutations
can occur anywhere in the DNA and in any cell. They are heritable only when they occur in the germ cells (eggs and sperm), but
mutations in the DNA of other (somatic) cells may lead to cancer. Even though
the DNA replication enzymes are very accurate, and there are also supplementary
systems for detecting and correcting damage, no system is perfect. We should,
therefore, recognize that some level of mutation is inevitable. However, the
mutation rate is increased by radiation, including ultraviolet light, and
exposure to certain toxic chemicals. We can, therefore, take some precautions
to minimize the risk.
The mutation rate for dogs cannot be determined
readily, but from indirect evidence and extrapolation from other species,
geneticists believe that mutation rates are normally on the order of 1 in
100,000 or less. For a sexually reproducing mammal, that would mean a new
mutation in a particular gene would likely not occur more often than once in
every 100,000 gametes. That may not seem like a high probability, but consider
that most mammals are estimated to carry 80-100,000 genes. This suggests that
every individual born has a good chance of carrying one new mutation in some
gene.
What happens to
new mutations?
Identical mutations are unlikely to occur simultaneously in the
same gene from both parents (probability: < 1 in 10 billion), so any progeny
will be heterozygous. (The exception being sex-linked genes, as the X and Y
chromosomes are not homologous.) Dominant mutations will be expressed and any that
are deleterious will be eliminated almost immediately from the population. If
the mutation is advantageous, and this advantage is noticed by breeder or
"nature", the mutation may survive and its
frequency gradually increase. If a mutation neutral, which is to say,
neither good nor bad (just different), its survival will be determined by
"genetic drift". New recessive mutations remain hidden from selection
until they reach a frequency where some homozygous individuals begin to appear.
However, this does not prevent drift loss, which doesn't depend on phenotype.
Drift is a consequence of the random nature of genetic events. For
example, if you breed a brown bitch to a black dog carrying brown, you would
expect ½ the progeny to be black and ½ brown, but probably wouldn't be too
surprised if you got 7 blacks and 3 browns in a litter of 10. It works the same
way for any gene that has two or more alleles. Suppose that we have only one
black dog (Bb), all the rest being bb. The one Bb dog may pass the B allele to
none or all of his progeny, or to any number in between. If he has more than 5
black progeny, the frequency of black will go up providing all contribute
equally to the next generation. In subsequent generations the frequency may
drift even higher, or back down.
In a large population, the frequency will tend to fluctuate by only
a small amount. However, small populations are inherently unstable and, if
other factors don't intervene, one allele will eventually take over. This is
called fixation. How long this takes depends on population size. With a rare
breed, fixation may easily occur within 25 generations (~ 100 yrs.)
Many recessive mutations persist for a few generations at low
levels before being lost again. Only very rarely do they reach a significant level
in the population (> 1 in 1000). In terms of estimates of genetic diversity
based on average heterozygosity, these genes are
effectively monomorphic, as a screen of 50 or 100
individuals from the population would generally fail to reveal any differences
for the majority of the these loci. When two individuals appear to carry the
same mutation, it may well be due to independent mutations. However, unless
there is some common ancestry, the chance of producing affected progeny should
be no more than 1 in a million. [Notably, in the first study of an "inborn
error of metabolism", Garrod (1902) observed
that "among the families of parents who do not themselves exhibit the
anomaly a proportion corresponding to 60 per cent are the offspring of
marriages of first cousins." He estimates that only about 3% of all
marriages are between first cousins.]
These estimates assume equal use of all individuals in the
population, and we all know how common that is. If a particularly popular sire
produces 10 times his "share" of sons and daughters, whatever
deleterious allele(s) he carried will get a substantial boost in the next
generation. A new mutation may be promoted from one-of-a-kind to moderately
frequent in this way. As long as we insist on making mate choice a popularity
contest, we risk introducing new problems as fast as we can develop tests for
the old ones.
Genetic
"load" and the founder effect
The human population carries at least 2500 deleterious mutant genes
(or, more correctly, alleles of genes) causing significant health problems. For
the most part they are fairly evenly distributed in the population. For the
entire Canis familiaris
population, the situation is likely fairly similar. Each individual is
estimated to carry a "genetic load" of three or four "lethal
equivalents", which implies recessive alleles that would kill of the
bearer if they were homozygous. As long as they are recessive, they should not
cause problems.
However, consider what happens if we form a subpopulation by
choosing 10 individuals from a much larger population. Though these individuals
will not carry the vast majority of the unwanted deleterious recessive alleles
found in the wider population, the few they do carry will be promoted instantly
from rare alleles (0.1% or less) to at least 5% in our example (or more
generally, 1/2N, where N is the number of founders).
Because random drift has a greater impact on a small population,
the population needs to grow rapidly, to at least several hundred breeding
individuals, so as to minimize the loss of valuable alleles. During this time,
we should select cautiously. While it is true that fixing "type" is
one of the prime objectives of purebred dog breeders, too rigorous selection
during the early generations increases the possibility of accidental loss of a
valuable gene closely linked to one of the genes under selection. Dalmatians,
for example, are all deficient in an enzyme required for correct uric acid
metabolism. The mutant gene appears to be closely linked to one of the genes
for the characteristic spotted pattern and was likely inadvertently fixed when
early breeders selected for that pattern (Nash, 1990).
Recognizing
mutation
Though, at an allele frequency of 5%, affected individuals should
only make up about 0.25% of the population, this would be a good time to stop
it from increasing further. However, would a mutation occurring at that
frequency be recognized as such? If we are talking about breed with average
litter size of four, then we are only looking at about one litter in 100 with
one affected puppy. If there have been no other reports, the breeder may simply
write it off as "one of those things". In a breed with larger
litters, the probability of two or more affected pups occurring in the same
litter is greater, but even in these cases, lack of exchange of information
between breeders and lack of education in genetics may result in a failure to
identify the problem as genetic.
Selection
Selection is only effective if we are dealing with easily
recognized phenotypes. However, undesirable mutations are not always that
accommodating. There is a full range of possibilities from silent mutations, that have no noticeable effect on proteins coded
for, to mutations that fail to make any functional product. There is even a
small possibility of improvement. Those, and the silent class, are no threat to
us. However, those that prevent normal function but do not eliminate it
completely are likely to present a substantial problem. One example is the vWD mutation in Dobermans. This mutation eliminates 85-90%
of the active clotting factor, but this low level is still sufficient to
protect a homozygous affected individual from excessive bleeding in most
situations. A dog that is "lucky" enough to avoid a major injury or
surgery may not be recognized and may even be bred. Consequently, the frequency
of the mutant allele rose to slightly over 50% in the population (Brewer,
1999).
This should not be regarded as an exception. Fewer than one in
three mutations appear to be fully lethal, and that the others cover the full
spectrum from the 0-100% activity. In addition to dealing with a handful of
easily-recognized genetic diseases in a breed, we are also likely to be dealing
with scores of others that reduce fitness but present no obvious phenotype that
can be used to identify them. If we can miss a gene that is only 10-15%
functional, how well are we likely to do with those that retain 80 or 90% of
their normal function?
Why should this
be a problem?
In a small population, drift inevitably leads to fixation for one
allele. Computer simulations show that if we start with a neutral allele with a
frequency of 5% in the population, as would be the case if it was originally
carried by 1 of 10 founders, it will be fixed 5% of the time (surprise,
surprise!). As the fitness of the homozygous phenotype decreases, its chances
of being the winning allele decline. At a 5% reduction in fitness, 3.5-4% will
still be fixed, most within 25 generations. At 15% the computer says the other
allele will almost always win - if our slightly deleterious allele gets no
boost from being linked to a selected gene or spread by a popular sire.
However, one or both these conditions are usually violated, as discussed above.
Furthermore, there is no guarantee that our selection will discriminate as
finely as the computer.
If each such gene reduced fitness by only 5%, and the effects are
additive, we could easily be facing a population with significantly lower
litter sizes, shortened lifespans and greater
susceptibility to non-genetic problems. Yet we would have no easily
identifiable gene to pin it on.
Conclusions
Longevity and fertility, commonly regarded as indicators of
"inbreeding depression", are reduced in canine populations which ave been inbred over a relatively short time period (Laikre and Ryman, 1991; Nordrum,
1994). However, most of the inbreeding in domestic dog populations does not
appear to be due to breeders intentionally mating close relatives1
(though there are certainly exceptions), but to the loss of diversity due to
drift and selection. The resultant loss of choices makes every individual a
close relative, no matter what breeding strategy is employed.
The outcome for any breed will depend on both luck and on the
breed's history. What is the effective population size? How many founders were
there? Over how long a period prior to the closure of the stud books had the
breed been refined? How intensive was the selection used to define type? Have
there been any bottlenecks? How strong an influence have popular sires had?
What can we do?
1. We can control many of the obvious genetic diseases by
supporting research aimed at locating the genes and developing direct DNA tests
for the mutant alleles. Test results should be employed to make certain that
carriers are only mated to clear individuals, rather than for wholesale
elimination of carriers, which would further impoverish the gene pool.
2. We can explain to breeders that
mutations will always be with us, and are not an indication of failure or bad
breeding practice, and that an open exchange of information will produce the
greatest rewards. We can also show them ways to achieve their personal goals
without making choices that are detrimental to their breed.
3. We can attempt to educate breed clubs on the importance of
maximizing diversity in the gene pool. As the keynote speaker at the recent
AKC/CHF conference, Dr. Malcolm Willis, pointed out, few breeds even have a
good idea of what their major genetic problems are, how many pups are in an
average litter, or how long their dogs live. Fewer still have any idea of how
to retain existing diversity or reduce the average inbreeding.
Notes:
1. Based on a study of
3 and 5-generation pedigrees of Australian Shepherds, Clumber Spaniels,
Standard Poodles and Malamutes.
References
Brewer, G.M. (1999) DNA Studies
in Doberman von Willebrand's Disease. Available online at: http://www.VetGen.com/vwdrpt.html
Garrod, A.E. (1902) The incidence of alkaptonuria: a study in chemical individuality. Lancet 2:
1616-1620. Available online at: http://www.esp.org/foundations/genetics/classical/ag-02.pdf
Laikre, L. and
Nash, J. (1990) "The Backcross Project" in The Dalmatian Quarterly, Fall 1990, Hoflin
Publishing Ltd.
Nordrum, NMV (1994) Effect of inbreeding on reproductive performance in
blue fox (Alopex lagopus).
Acta Agriculturae Scandinavica, Sect. A, Animal Sci.
44: 214-221.
Copyright 1999 John B. Armstrong. Reprinted
with permission. It may be
reprinted providing it is not altered and appropriate credit is given.
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