Breeding a PKD Positive to a PKD Positive
by David Brinicombe
Many PKD experts suggest that breeding a PKD positive cat to a
negative cat should be approved, with safeguards, in order to breed a
replacement negative cat. This is to prevent damage being done to the
breed gene pool.
They also advise that positives should not be bred to positives.
However, this may be appropriate in order to preserve a good type if
both cats have genes which are valuable to the breed gene pool.
It is possible that a replacement cat could be bred with less positive
kittens being produced than by mating each cat to a negative cat.
I decided to have a look at what might happen if this was done.
Average statistics give 17% more positive kittens will be born,
equivalent to 1 kitten in a litter, but statistice do not work well in
small numbers.
I decided to do an experiment using real random data produced by
tossing a coin.
Why would a breeder want to mate a positive to a positive?
Consider a small country with a small Persian gene pool. A breeder has
been working towards producing a Lilac Colourpoint Persian. She has a
Seal Colourpoint and a Lilac carrying colourpoint, but has responsibly
tested them first.
Unfortunately, by Murphy's Law, both are positive for PKD. Alternative
cats are not available, and if a cross is not permitted, she is
virtually back to square one with her breeding programme.
The numbers experiment
A coin was tossed 12 times and the results written down as 6 pairs of
tosses. This corresponds to a litter of six kittens. This was done
once and once only.
HT;HH;TT;HH;TT;TT
The first toss of each pair represents the first parent with one PKD
gene and the second toss the second parent with one PKD gene. There is
an equal chance that each will pass on the gene.
Heads and tails were not defined as with or without the PKD gene
before the experiment, so we have two sets of results.
HT;HH;TT;HH;TT;TT
+ X - X - -
or
+ - X - X X
Where:
+ is positive
- is negative
X is homozygous (lethal)
It is usually accepted that the homozygous (two genes) form of PKD is
lethal, so the kittens marked X will not be born or not survive. We
get 1 positive kitten and either 2 or 3 negatives.
If the second parent is negative, we strike out the second toss of
each pair.
H_;H_;T_;H_;T_;T_
+ + - + - -
or
- - + - + +
3 positives and 3 negatives either way.
If the first parent is negative, we strike out the first toss of each
pair.
_T;_H;_T;_H;_T;_T
- + - + - -
or
+ - + - + +
This result is only one kitten different giving 2 positives and 4
negatives or 4 positives and 2 negatives.
Conclusion
This esperiment can be done several times and each time it will
represent a potential litter of 6 kittens. Any result is possible,
including all positives or all negatives, but usually we would expect
some positives and some negatives from either a positive to positive
mating or a positive to negative mating.
The differences between litters is likely to be as different as
between the two types of mating. In practice, positive to positive
matings could be just as successful as positive to negative matings.
Either way, and mating of a PKD positive cat is likely to produce
positive kittens and provision must be made for the welfare of these
kittens.
last updated 16 May 03: