Inbreeding

["guest@endurance.net" <guest@endurance.net>]

K S SWIGART   katswig@earthlink.net

> Perhaps it makes sense to us because we pay attention to the 
> quality of the animals we are inbreeding.   As should ANY breeder pay 
> attention to the quality of the animals to be bred (and I will admit, 
> not all breeders do this).    Also, the situation being discussed in 
> the previous post is called "linebreeding" by most breeders; 
> "inbreeding" is father/daughter, mother/son, full-siblings.   

In the Thoroughbred world, any horse is considered to be inbred if it 
has the same horse more than once in its four-cross pedigree.  In TB 
parlance, the foal resulting from the previous mentioned cross would be 
inbred 2x3, but if people want to call it linebreeding to make them feel 
better about not inbreeding because the word has negative connotations 
to it, that is fine with me. But, no matter how you slice it, breeding a 
daughter to a grand son is breeding closely related individuals.

> So, in the breeders' use 
> of the terms, both inbreeding and linebreeding, done wisely, work 
> quite well. The rule of thumb is that you get what you breed: breed 
> "good to good, you get good; breed bad to bad, you get bad".   (This 
> is not to say an occasional recessive gene won't crop out---but can 
> you say that won't happen with "regular" breeding?)

The genetic philosophy behind inbreeding (or linebreeding or close 
breeding or whatever you want to call it) is to increase the incidence 
of homozygosity (and if you want me to do the math to show why this is 
the case I can--but it is a bit tedious and a bit difficult to follow).  
The idea being that with homozygosity, future generations will breed 
"truer" (i.e. more like their parents), but additionally, since many 
negative traits have a tendency to be recessive traits (and therefore 
need to be homozygous in order to express themselves), the practice of 
inbreeding is specifically intended for the purpose of getting those 
negative recessive traits to express themselves and therefore you can 
remove them from your breeding herd (i.e. culling).  With the 
understanding that not only to the poor quality foals have to be culled, 
but the parents who threw them need to be culled to, and any other 
offspring of either of those parents need to be "tested" to see if they 
are also "carriers" of these negative traits.

Whatever the case may be, if you don't understand that breeding more 
closely related individuals doesn't increase your chances of "an 
occasional recessive gene cropping out" over outcrossing, you don't 
understand the underlying philosophy of breeding closely related 
individuals.

Yes, inbreeding (even breeding father's to daughters, mother's to sons, 
and full siblings) can lead to some exceptional individuals (and the 
whole idea behind "purebred" is that the horses are somewhat inbred with 
the undesirable traits having been removed from the "breed").  But it 
has to be done by people who know what the hell they are doing (i.e. 
understand the underlying philosophy) and know what traits they are 
trying to set and which ones they might be trying to remove from their 
breeding herd.  And there are some stud farms that have practiced it 
intelligently for many generations and therefore are in possession of 
some truly exceptional individuals that breed on those truly exceptional 
traits to their offspring.

However, the fact is that ALL horses have some genetic faults—even if 
they aren't recessive traits, and if you regularly practice inbreeding 
(unless you cull ruthlessly) you are going to SET those faults in your 
herd (that is the whole idea behind inbreeding). The more closely 
related the individuals in your herd, the less genetic diversity there 
is in your herd, and THERFORE the less opportunity there is for 
improvement.

Additionally, the more closely related the individuals you breed, the 
higher your chances of and unpleasant recessive trait expressing itself.  
Anybody who chooses to do it for a one off foal is an idiot--or just 
ignorant.  And if you are going to do it, you need to find out as much 
as you can about the horse that you are breeding most closely.

From my own observations, many of the people in the Arabian world who 
are practicing inbreeding either a) don't understand the underlying 
philosophy, b) are selecting for what I consider to be the "wrong" 
traits, or c) do it because over time most of their herd is related to 
each other and they already have too many horses, they aren't going to 
go out an get more.  And they love all their horses and think that they 
are all perfect so they aren't going to admit that they may be 
something less than "breeding quality."  Not all, but many.

For me, my best broodmare is/was the mother of my breeding stallion 
(because my best broodmare throws stallion quality foals--she also 
happens to be the mother of a filly I have that will probably make a 
nice broodmare some day, but I won't be breeding her to her brother). 
Since I do not have the resources to breed tons of foals, I had to 
decide which to keep, the mare or the stallion.  For sentimental 
reasons I kept the stallion, so I gave away my best broodmare to 
somebody who could use her to get a very nice foal from her...NOT by my 
stallion.  

Do I think I could have gotten a very nice individual by breeding my 
stallion to his mother?  Maybe, but the risks associated with doing 
that are much higher than if I choose to out cross, and I just plain 
old don't have the resources to be a responsible inbreeder (either 
financial or emotional), and, I contend, very few people do.

kat
Orange County, Calif.

p.s. Yes, it has been scientifically demonstrated in an assortment of 
species in which the effects of inbreeding can easily be monitored 
(ones with shorter generations and larger "litters" than both people 
and horses so the effects can be monitored in a much more controlled 
environment), that it does have a tendency to lead to neuroses (I might 
be able hunt down the specific studies, but if I have to hunt too hard, 
I won't bother).  So, unless you specifically select against neurotic, 
you are more likely to end up with it in your inbred herd.  Very few 
inbreeding programs that I have observed select against neurotic, and 
in this particular instance, a "crazy" horse is what we were talking 
about.