Hot Topics from Horse Breeders Forum August 2010

Inbreeding and line breeding

this thread originally started when a pedigree was posted of a mare...this is how it developed....

poster 1
I will be honest and say that mum's pedigree made me cringe. Do you think the breeders know what they are risking when they inbreed so tightly? I often think that people who breed pedigrees like that cannot understand basic genetics. Hope that doesn't come across as rude but the risks of recessive genetic diseases is so high when one ancestor (not mum or dad) represents 55% of the genetics of an individual. Scary!

Regarding inbreeding it is estimated that all mammals carry at least 3 mutated genes capable of causing a severe genetic abnormality if passed on by both parents. The closer two parents are related the more likely it is that they share genes so the risk of hideous diseases like SCID (found mostly in Arabs), HERDA (found in quarter horses) Junctional Epidermolysis Bullosa (in Belgian warmbloods) HYPP (slightly unusual in that it can cause disease when present in just one dose and found mostly in quarter horses) the "new" genetic disorder in Fell ponies are all magnified if you inbreed. An interesting thought is that over 95% of human genetic disorders are so rare they don't have a name and there is no knowledge of their prognosis. I'm sure the same is true for horses.

I'd be interested to know about what this stallion's fertility rate was when used on closely related mares because if he carried a mutation that was so awful it was incompatible with life the foetus would be miscarried early in the pregnancy and it would look as if the mare had not taken resulting in low fertility rather than offspring with genetic diseases. I know lots of breeders have done "tight line breeding" and had good results, in the show ring at least. In competition breeding very close line breeding has not been as successful mostly because it leads to decreased general well being and decreased hardiness so they don't stand up to the rigours of competing the same way that horses who are more genetically diverse do.

As you say sometimes using an inbred stallion or mare can be useful but IMO only if used on an unrelated horse. I used an inbred stallion last year on my ID x TB mare to help give me better predictability on how the foal would turn out as my mare's genes are rather diverse. The stallion I used is tightly line bred 2x3 but is a little unusual in that he had an excellent competition record himself, he competed internationally up to Inter I level. His offspring have done even better than he did so he seemed a good choice. The foal turned out very nice and my mare is back in foal to the same stallion for next year. Only time will tell if the foal turns out to be everything I ever dreamed of though

Responses

poster 2

Sorry to hijack the thread and turn it into a discussion on inbreeding (but not so sorry that I can resist doing it)- but I do feel that this is a simplistic and unhelpful condemnation of inbreeding. Far from being unable to "understand basic genetics" I suspect this successful breeder knew very well what she was doing and what she might be risking and armed with this knowledge went on to produce, by the judicious use of inbreeding, these top class animals. The proof of the pudding, and all that; this is a very nice family, honed by inbreeding into what I suspect the breeder wanted. Of course inbreeding has dangers, but I do feel it shouldn't be treated as a bogey and condemned out of hand, even in its more extreme forms.

To begin with, the need for inbreeding and its likely effectiveness does depend on the population you started out with. A diverse population actually demands inbreeding if you are aiming for a particular type or attribute. Selection for performance can produce and accelerate inbreeding whether the horse is bred to show or bred for some other discipline; its simply not true to say that inbred animals cannot compete successfully; rather the opposite, in fact; without inbred animals we would not have animals with the attributes of sports horses.

As ever its interesting to look at the TB in this instance; however I suspect the TBs early story will be mirrored by just about every other breed of horse that you care to investigate. The TB as a breed has grown and developed from relatively few original animals, with a few additions down the years; inbreeding has so developed the breed for its intended purpose that it looks likely to remain inbred; outcrosses just aren't fast enough. However, there are lots of TBs; so more chance, I suppose, of variety through mutation or genetic drift (sort of accidental inbreeding fixing otherwise rare characteristics in isolated populations). You maybe wouldn't need or wish to use extreme inbreeding with TBs nowadays; but pulling out at random one famous name from the past; Eclipse: http://www.pedigreequery.com/eclipse

quite a lot of inbreeding there, some obvious and colour coded, but Hautboy, for example, occurs a few more times whre you can't immediately see him; and of course, Eclipse was a real performer; THE racehorse of his day; "Eclipse first, the rest nowhere". So inbreeding didn't do him any harm; to a large extent it created him.

However this inbreeding is done with horses already inbred; a bit further back, the inbreeding was even more intense; look a few generations back to Eclipse's great-great grandam Betty Leedes for example: http://www.pedigreequery.com/betty+leedes

and we see a similar pattern to that appearing in the arab pedigree detailed earlier; Spanker mated to his own mother and the grandchild from this mating mated to Spanker's son.

I would imagine that the assimilation of different bloodlines demanded inbreeding as a way of fixing a variable type. The inbreeding, extreme as it is, is still offset by (presumably) totally unrelated outcrosses but as the TB develops these, of course, vanish.

I suppose what I'm trying to say, is- inbreeding is a tool; a powerful and necessary tool; it has a downside in reduced vigour if continued beyond a certain point; but none of us really know what that certain point is; it varies between bloodlines and animals. The flip side of the coin is the advantage of being able to harness hybrid vigour by outcrossing. Again, just as you increase the possibility of doubling up deletarious genes by inbreeding, so you also increase the possibility of doubling up on advantageous genes; and IF an "if" appears it can be dealt with. It cannot be assumed that an "if" WILL appear. Meanwhile outcrossing can spread unwanted genes through a wide population to surface later in matings between apparently unrelated individuals.

Of course, we are lucky; we have far more genetic knowledge available to us as lay-people than the most eminent scientist would have had just 50 years ago, and old breeding plans can well represent a lack of knowledge which WE WOULD HAVE SHARED had we lived at that time. But with this relatively recent breeding plan; I think the breeder knew what she was doing.

poster 1

I agree with most of your post but would differentiate between inbreeding and line breeding and not just the tongue in cheek difference that it's inbreeding if it doesn't work. I don't think there's a hard definition of inbreeding or linebreeding but I tend to think 3x3 and further away is line breeding, 2x3 is teetering on the edge of inbreeding and 2x2 and closer is inbreeding. I fundamentally disagree with inbreeding in it's extreme form.....

http://www.upei.ca/cidd/intro.htm

I'm currently analysing the pedigrees of the top eventing stallions of all time. What is very noticeable is that although most of them are TBs where line breeding and inbreeding are very common none of the top stallions are line bred closer than 4x3 and most of them are not line bred closer than 5x5. Having gone through 50 stallions now I don't think it is a coincidence that inbred stallions are not on the list. I'm not saying that line breeding has no place in a breeders plans. Clearly line breeding has been an essential tool for producing the horses we have today. What my opinion on it is that, like most things, it can be done too much and cause disease and failure to thrive. I would certainly consider line breeding in the future, I was looking at a stallion who would allow me to line breed 3x3 to a stallion. However since doing the work on the eventers pedigrees I've changed my mind. For me 3x3 is too close. I might look at 3x5 or 3x6 instead and see what I get.

Another reason why I dislike the extreme inbreeding shown in the Arab mare's pedigree is that horse breeds with the most inbreeding do have the highest incidence of genetic disease. Arabs are number 1 in this list. Yes I know that sometimes extreme inbreeding works but frequently it doesn't and it is the horses that it didn't work for that suffer.