Re: [pyrnet] Recessives and carriers

["Linda Weisser" <lmweisser@olywa.net>]

----- Original Message -----
From: darrell goolsbee <dgg@star-telegram.com>

I would be interested in the explanation for this, as I'm not sure I
understand. If you are breeding known non-carriers, how are you
continuing to pass along the problem?>>

I'll send you the article when I can dig it out.  I am now confused since I
was addressing the issue of breeding a dog descended from a known (or
obligate) carrier and not breeding from known non-carriers.  Of course, if
you breed from *known* non carriers, you are not going to pass anything
along.  One can "change" a descendant of a known carrier (50% risk) into a
known non-carrier by test breeding.  I am very familiar with one such case.
In this case someone I knew had a young male that she had bred a couple of
times.  After that his sire produced a litter with dwarfs and of course,
was an identified carrier.  So, the son had a 50% chance of carrying, but
so did his son and also his son's three daughters who were in breeding
programs but had not yet been bred.  The dog in question was not bred
again.  Nor, of course, was his sire.  Some years later the owner of the
questioned dog was able to lease a bitch who was an obligate carrier,
having produced dwarfs herself.  This young male was bred to this bitch.
She whelped (thank god) *12* pups all of whom lived and none of whom was a
dwarf.  This "cleared" the male to the level of 96.8% which is generally
considered safe.  In clearing this dog, the owner also cleared his son and
the three grandaughters as all of the animals to whom these dogs were bred
came from dwarf free lines.

How were those 3 grandaughters identifed as at 50% risk?  Do it step by
step.  The dwarf producer (male #1) is a known carrier.  Therefore his son
had a 50% chance.  (By the way, the son's dam was bred to this known
carrier twice before he produced his first dwarf and she had 14 pups in
those two litters which effectively cleared her)  So, now the son who has a
50% chance of being a carrier is bred.  Presume that this dog *is* a
carrier.  The bitch is not.  In that litter sired by him half of those pups
will be carriers.  Breed a carrier to a non carrier and you get all
"normal", half carriers.  So, now you take a son of the possible carrier
(son of obligate carrier).  *If* the sire (male #2) is a carrier his son
has a 50% chance.  Now, presume that the son (male #3) *is* a carrier and
is bred to a non carrier.  His 3 daughters mentioned above also will have a
50% chance.  As in carrier to non carrier produces 50% carriers.  Can you
see how the risk never changes no matter how many breedings to non carriers
are made?  Sometimes it helps to take a pedigree and just make marks on it
to help you follow the line.

Nonetheless, dilution is not possible.  Which is why it is absolutely
mandatory that any buyer of any posssible breeding animal which has an
obligate carrier in its pedigree, no matter how far back, be told of this
risk.  In my view, to do anything else is unethical.  If, as in the case
above, the intervening dog has been test bred clear then this is not
necessary.

Linda