"A breeder with a large scale breeding program can discover genetic defects more rapidly than a hobby breeder who has only a litter or two a year, without the danger of having to scrap a whole breeding program because of it."
Is this a valid theory? What are the implications and secondary effects to the gene pool?
Both stud dogs are the same age and retain normal health. Each produces puppies of sufficient quality that one in four are bred. No descendant is bred prior to the age of 2 years, and in turn one in four in each succeeding generation is added to the gene pool.
Each stud dog carries a simple recessive gene for a late onset disorder. In the first year each meets a carrier bitch. One in four puppies develops the disease and is diagnosed at age 5. Thus, both breeders learn of the defect in year 5 of their breeding program.
Stud A is bred to 10 bitches each year, with 4 puppies in each litter. His offspring enter the breeding program at 2 years of age, as do the subsequent generations of descendants. One in four is bred, each producing a litter of 4 (breed average) as the years progress.
Year F1 F2 F3
2000 40
2001 40
2002 40 40
2003 40 80
2004 40 120 40
--- --- ---
200 240 40 = 480 descendants
# who inherit 50% 25% 12.5%
defective gene 100 60 5 = 165 carriers
40 potential carriers of breedable age have entered the gene pool in a 5
year period.
Stud B is bred to 2 bitches each year, with 4 puppies in each litter.
His offspring enter the breeding program at 2 years of age, as do the
subsequent generations of descendants. One in four is bred, each
producing a litter of 4 (breed average) as the years progress.
Year F1 F2 F3
2000 8
2001 8
2002 8 8
2003 8 16
2004 8 24 8
--- --- ---
40 48 8 = 96 descendants
# who inherit 50% 25% 12.5%
defective gene 20 12 1 = 33 carriers
A total of 8 potential carriers have reached breeding age and have entered
the gene pool.
That the more prolific breeder had more dogs to evaluate for the defect is
irrelevant. Whether there are 4 puppies or 400 born in that first year -
all age at the same rate!
The only notable difference is that owner of Stud A has spread the
defect considerably further, and may have produced many more affected offspring
for purchasers to contend with.
What if the small breeding program does not produce an affected for 10
years? Dogs who are used less have less potential to meet a carrier bitch
early in their stud careers.
Question: How does this impact on the breed?
In 10 years of breeding there will be 96 breeding age decendants who have
the potential to carry the defective gene from Stud B.
Year F1 F2 F3 F4 F5
2000 8
2001 8
2002 8 8
2003 8 16
2004 8 24 8
2005 8 32 24
2006 8 40 48 8
2007 8 48 80 32
2008 8 56 120 80 8
2009 8 64 168 160 40
--- --- --- --- ---
80 288 448 280 48 = 1144 descendants
% inheriting 50% 25% 12.5% 6.25% 3.125%
defective gene 40 72 61 17 1 = 191 carrier
However, it is not uncommon of for a prolific sire to remain undetected as
a carrier of a genetic defect for many years, through lack of disclosure
or failure to meet a carrier bitch until later in his stud career. Let us
examine the math for Stud A in this circumstance and how it spreads through
the gene pool.
In 10 years of breeding there will be 490 breeding age decendants who are
in the gene pool with the potential to carry the defective gene from Stud A.
Year F1 F2 F3 F4 F5
2000 40
2001 40
2002 40 40
2003 40 80
2004 40 120 40
2005 40 160 120
2006 40 200 240 40
2007 40 240 400 160
2008 40 280 600 400 40
2009 40 320 840 800 200
--- --- --- --- ---
400 1440 3680 1400 240 = 7160 descendants
% inheriting 50% 25% 12.5% 6.25% 3.125%
defective gene 200 360 460 87 8 = 1115 carriers
This final example represents quite closely the spread of genes by
breeding program that produces an average 10 litters a year, selling open
stock and offering 3 or 4 studs to outside bitches.
While such a breeder may be using more than one
sire, it is common practice to cross the descendants of one line with
another within breeding kennels, and as a result, it is rare that such
sires do not end up in the pedigrees of nearly all of each other's
descendants after a few generations.
So, to return to the first questions: Does the large scale breeder
have an advantage over the small scale breeder?
...Not for the first 5 years.
Does the policy allow the large scale breeder to continue without
scrapping all of the breeding program?
...No. The risk that a
descendant is carrying the defect is determined by the number of
generations that separate him from the last known carrier, not by the
number of dogs in that generation. Each dog in the F1 generation carries
a 50% risk, each in F2 carries 25% risk, each in F3 carries 12.5% risk.
The odds of selecting a non-carrier from any subsequent generations from
which to continue a breeding program by chance (the only method
available) are identical for both breeders, whether they have 2 dogs to
choose from or 200.
The only significant difference between the two breeding programs in the discovery and
control of a recessive defect, is in the speed that the defect is spread
before it is discovered. The large breeding program far outstrips the
small one in this regard.
[note]
In any real-life setting, the specific numbers and averages would vary,
linebreeding may result in variable risks in descendants, etc.
However, for the purposes of comparison - using static criteria is required
to illustrate the relative usefulness and potential
damage to the gene pool of the two breeding philosphies applied under identical
circumstances.
(c) Catherine McMillan