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> What would be best scenario, of course, Heidi, would be for every
> stallion owner to test their stallion and geld if he is positive. That
> would eliminate the bulk of the problem.
No, Bette, that is not what the geneticists say is the best scenario.  If
all positive horses were eliminated, we would lose roughly 10% of our gene
pool, and would most likely shift it sufficiently to bring more lethals to
the forefront (such as lavendar foal, cerebellar abiotrophy, and perhaps
some not even yet known) for which we have no genetic tests.

The best scenario is to test, and then to treat SCID carrier status like
any other fault, so that the frequency of the gene in the population is
gradually reduced over time.  In this way, the millions of other genes
carried by horses that happen to carry this one faulty gene are not lost
to the gene pool.  The ability to test essentially disarms the
disease--there is no longer ANY excuse to produce an affected foal.  No
carrier stallion should EVER be bred to a carrier mare, and all offspring
of carriers should be tested to see if they are carriers or not. 
(Remember, HALF of them will be clear!)

Also, the best scenario involves honesty.  The problem with the pressure
from folks who would geld all the carriers is that it causes people to
hide the carrier status of their stallions.  If folks would listen to the
experts in the field of population genetics and use testing as a
management tool, as it SHOULD be used, there likely would be less folks
unwilling to disclose the carrier status of their stallions.

I hope the QH folks realize that they don't need to flush all the Poco
Bueno horses down the drain, either--in all likelihood, given the state of
genetic technology today, they, too, will have a carrier test in the very
near future.  And if the quality of the individual carrier merits being
bred on, I hope they will likewise have the sense to manage the disease
and reduce the gene's frequency over time, instead of just jettisoning
some of their best breeding stock.

It took hundreds of years for SCID to become entrenched in the Arabian
population--it doesn't make sense to try to eliminate it overnight. 
Likewise, the information about the "Poco Bueno thing" states that as many
as 5% of the pedigrees don't double Poco Bueno himself but rather go back
to his ancestors, so clearly that mutation didn't arise with him, either,
but rather further back.  So they shouldn't expect to solve the problem in
one generation, either.

But again--thanks to genetic testing, one need never produce a SCID
affected foal even if one DOES judiciously breed a carrier individual (why
is it always the great hue and cry to geld the stallions and ignore the
mares, BTW?), and in the near future, the same should be true of HC in
QHs.

Heidi


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Prudence and focus will carry you a long way on a horse. 
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