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> Assuring that you do not produce an SCID foal is the starting point.
I
> stand by my assertion that if you are breeding to get a great using
horse,
> of any breed, there are enough stallions standing in this country that
you
> can find a negative one just as suitable for your mare as the positive
one.

However, if it is your mare that is a carrier, and you love her and you
admire all her fine qualities and you want to replace her with her foal,
and you intend to keep the foal for its entire life because you want to
have a part of your beloved mare, there is absolutely no reason that you
shouldn't breed this mare....because, as you say, there are enough
negative stallions out there that are suitable for your mare.

And, believe it or not, ALL horses are genetically unique (I have never
heard of identical twins in horses; although I do recall hearing reports
of a clone having been born last year).  If you want a foal from THAT
individual, then you want a foal from that individual.  One that shares
some of the same ancestors a few generations back, no matter how closely
related, just isn't the same.  And there is nothing either ethically or
morally reprehesible about producing a foal whose only flaw might be
that it carries a gene for a trait that will never be expressed.

It is the height of foolishness (from both a practical and a genetic
standpoint) to remove otherwise high quality individuals from the gene
pool for a trait that, because of the ability to test for it, need never
be expressed.  And it will never get to the point that the carrier
status is so prevalent that non-carriers are impossible to find, since,
by definition, those horses that are homozygous for the trait do not
survive to reproduce, consequently, supposing that people do not breed
carriers to carriers, at least HALF of all horses born will be
non-carriers.

The test for SCID carrier status has made the genetics of the disease
irrelevant, since it is now possible to never produce an affected foal,
one can breed carriers to non-carriers with 100% assurity of not
producing any foals with this genetic defect. Because of the test, it is
no longer necessary to remove carriers from the gene pool, and anybody
who says otherwise doesn't know anything about the genetics of heritance
or statistics.

And these statements are true for all simple recessive genetic traits
for which there is a simple test that will tell you if a particular
individual is a carrier, especially if it is a trait that kills any
homozygous individuals within the first few months of life.  This does
not apply traits that do not express themselves until later in life (so
that homozygotes have the opportunity to pass on the defect to ALL of
their offspring), nor does it apply to traits that are genetically
dominant, or are additive.

How and to what extent one removes traits such as these from the gene
pool is quite different from a trait that doesn't express itself at all
in horses that are mere carriers.

And wise breeders would do well to inform the entire industry that not
only is not necessary, but not desirable to remove all carriers from the
breeding population, since people will be more likely to seek and
provide honest and accurate information (so people can make better
breeding decisions) if they know that their breeding stock won't be
automatically disqualified if carrier status is divulged.

It is the fear that some people might make the totally irrational
decision of never breeding to a carrier, no matter the carrier status of
their own horse, that will discourage the owners of bloodstock of doing
the responsible thing of testing their bloodstock before using it.

The idea of "only breed non-carriers to non-carriers" will do more harm
than good to the breed, especially in a closed gene pool.

kat
Orange County, Calif.



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