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Line breeding is generally not desireable because it eventually can
result in decreased hardiness or vitality of a breed, and it can tend to
magnify undesireable genetic characteristics.
The hardiness or vigor can be bred right out of animals with repeated
line breeding or especially inbreeding. I don't believe we (scientists,
geneticists, etc) know yet why this occurs; We just know it does.
Conversely, we have all heard of "hybrid vigor", the extra hardiness that
results from a cross between different breeds. I believe Fjords have a
higher innate level of hardiness or vigor than many other breeds, but
that this could be lost if Fjords were started to be linebred to a large
degree.
Linebreeding can also magnify unwanted to genetic characteristics, even
without us knowing it's happening. Let's say, for example, that there
was a very good stallion who was not parrot-mouthed himself, but in his
genes had the tendency toward that problem. Someone decides to
linebreed him, meaning he is bred to some of his own granddaughters (one
way to linebreed). Those resultant offspring are still fine, i.e. are
not parrot-mouthed, but the tendency has been magnified because there is
now twice as much of that stallion's genetics involved. Now let's say
these are bred to some Fjords who also have a latent tendency toward
being parrot-mouthed, and some parrot-mouthed offspring show up. Now
here's the problem; It's possible that none of the Fjords that were bred
to each other in the above example were parrot-mouthed. Yet finally the
circumstances became such that some offspring afflicted with that problem
began showing up. Also, we are now three generations away from that one
stallion, and many more Fjords have entered the picture, making it very
hard to know which horse or horses are to blame. It's likely to be only
a few offspring affected, rather than a widespread "epidemic". Finally,
since the offspring are likely to be good in other respects because they
come from that very good stallion, it is going to be very tempting for
the breeders to overlook the parrot-mouthedness and continue breeding
them anyhow.
And I don't know where the thing about getting either "a real good
offspring, or a real dud" ever started, but it just ain't so folks. Like
Joel, I hear it all the time as well. The reality is though, it is
likely to be pretty much an average horse like any other. I can cite a
number of examples. Let me use my neighbor's cow herd to illustrate.
Before they started using a veterinarian (my boss) much, they routinely
practiced inbreeding and linebreeding. In their case it was not with any
specific objective, rather it was due to ignorance and complacency. They
didn't want to spend the money to buy a different bull, so they just let
the bull continuously run with the herd and he started breeding his own
daughters, etc. Before long they started having lots of small calves,
not really vigorous, and most of them had "contracted tendons". The legs
would straighten out after a week or two with some splinting, and the
calves took a little more nursing care, but after a while you couldn't
tell anything was ever wrong with them. And that's just about how it
would go with horses as well. Overall they would just have mild defects
externally, but the hidden danger would be internal: a tendency toward
small, less vigorous offspring with slight skeletal defects and lowered
immunity or mental capacity.
Despite all the above, some people will still try to defend linebreeding
because, once in a while, you do get a superior animal. The thing they
forget is that once in a while you also get a superior animal without
linebreeding. And with linebreeding, there's more potential for damage
to the breed than there is for improvement. We have enough different
Fjord stallions in this country now that it is not that difficult to find
one with whatever characteristic a person is looking for, so linebreeding
is just not justified.
Finally, how do you tell the sex of a chromosome?
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Pull down it's genes!
Sorry, I couldn't resist. I can hear the collective groan already! : )
Brian Jacobsen, DVM
Norwegian Fjordhest Ranch
Salisbury, North Carolina
