Chondrodysplasia Impact on Height

[Kirk- Cascade Herd US]

Here's a theoretical question for those of you with experience with chondrodysplasic (short) Dexters.

Imagine if you had a large herd of 100 chondrodysplasic cows and you bred them all on the same non-chondro bull for a period of 10 years. Let's say you had 1000 calves over that period of time and that each cow gave you 5 chondros and 5 non-chondros. Let's say you measured the height of each calf at 3 years of age and you used the data to attempt to compute the average effect of the chondrodysplasia gene on the calves compared to the heights they would have been without the gene. You especially noted the average heights of the non-chondro and chondro calves out of the same pairings.

Just based on your own personal observations over the years, what would you predict the average height impact to be if you conducted such a test? Would you guess an average height difference of 4 inches? 5 inches? 6 inches? What's your guess?

Another way of thinking of this, imagine if you could take identical twin calves very early in their cellular development and insert a chondrodysplasia gene only in one of them. What would you predict the height difference to be, between these calves?

Just curious.

Kirk

[Mark Bowles]

Kirk
Im a bit slow on this one, what are you suggesting exactly.
Mark

[Kirk- Cascade Herd US]

Kirk
Im a bit slow on this one, what are you suggesting exactly.
Mark

The chondrodysplasia gene has an expected effect on Dexters, much like the polled gene has an expected effect. While the polled gene affects horn growth, the chondrodysplasia gene affects skeletal growth. A single polled gene reduces horn growth by 100% (ignoring possible scurs). A single chondrodysplasia gene reduces skeletal growth, but not by 100% (or you would have no calf at all). What would you guess is the average percentage reduction in height as compared with what the animal's height probably would have been if it had not inherited the gene.

Without a large scientific study, you can only guess the average impact based on subjective evidence. I've heard perhaps a 10% to 15% reduction in height. In inches (we didn't invent inches in the USA, but we seem to be stuck with them), a chondrodysplasic bull with otherwise 46 inch genetics might be reduced by 5 or so inches.

If we could magically go back in time and and simply change your cows' chondrodsyplasia genes to non-chondrodysplasia, how much taller would they be today on average? Just looking for your guesses based on a gut feel.

Hope this is more clear,
Kirk

[Woodmagic]

You have started something Kirk! I would suggest it isn’t as simple as you suggest. When I began I had animals from 36” to 46”. They have grown since then and I have come across animals at least 48” and I suspect more, where the motive is beef at any price. Particularly where a long leg bull is used on short leg cows, and the eye is only on the short leg.
I retained all my animals, so had some opportunity to compare the long legs with their short leg relatives. If I could unearth them, I took measurements of the lot.
I concluded that there were two influences on size. The bigger ones were bigger sometimes in both the dwarf and the long, and followed in families, the biggest variation came in the very big. The one group by families varied by only a couple of inches – very roughly between 38” and 40”, but the bigger ones could give a vast difference down from 46” to 36”. I concluded that this was probably the original cross possibly Devon, which had introduced the bulldog, still operating. Mudstopper and Mallemuck (full sisters out of related parents, were in the medium range, 38” and 40”) and by using the son of Mudstopper his progeny steadied up on size almost immediately, which made me decide I was on the right lines. My herd today is roughly 40” a size commonly found when I came into the breed, amongst the achondroplasia free.

[Kirk- Cascade Herd US]

You have started something Kirk! I would suggest it isn’t as simple as you suggest.

I think I'm hearing that the single chondrodysplasia/ achondroplasia gene hides the animal's true size genetics the way the polled gene hides the animal's true horn genetics. It is very likely that there are many genes for horns affecting size, shape, color, etc.. The polled gene is likely a single gene that simply turns off the horn growth, yet leaves all or most of the true horn genes in place. So it would be fair to ask of a polled cow, "what are its horned genes like?", especially if one planned to breed any horned calves out of this polled cow. One could make a good guess at the polled cow's horn genetics by looking at its horned siblings/close relatives - do they have straight horns?, long/short?, upswept horns?, etc..

I think I'm also hearing that the non-chondro offspring from a chondro cow can give you a good idea of likely size genetics hiding in a chondro (short) animal. I think I'm also hearing that animals with larger genetics on average (but not always) have greater shrinkage (perhaps as much as 9 or 10 inches in some cases) due to the gene, and that animals with shorter genetics from a truly shorter line, have less shrinkage on average (but not always), when affected by the single chondro gene.

It seems like breeders would have to be careful in using chondrodysplasia affected animals in selecting for size or they might accidentally create a line of giants (hidden by the gene). Do most chondro-breeders track the sizes of their non-chondro calves to do a reality check? Do any of your associations speak to this difference in the height standards? Do any of your show judges take this into account?

Maybe I'm blundering into some sensitive questions, but I'm just interested in the science I've been told that my focus on science blinds me to sensitivity, but it's all so very fascinating.

Kirk

PS. Do the Woodmagic animals of small stature with truly small genetics seem to be overall smaller, or is the shortness found mostly in having shorter legs?

[wagra dexters]

We have 12 Hedgehog daughters, and as many other Hedgehog relatives. I can not reach to AI them with the bottom half of the inside tailgate shut, but I can have it shut, if need be, for the taller red cows. The Woodmagic line is proportionately smaller, in our stock.
The smallness is obviously a genetic trait, rather than from any inbred argument, because the hybrid, and subsequent generations, are still a lot smaller, from the most extreme outcross line.
Margaret.

[Kirk- Cascade Herd US]

The (Woodmagic) smallness is obviously a genetic trait, rather than from any inbred argument, because the hybrid, and subsequent generations, are still a lot smaller, from the most extreme outcross line.

One of the problems with breeding chondrodysplasic animals in an attempt to achieve a short statured herd is that the chondros cannot breed true. Half of the calves are going to be larger. Since the Woodmagic lines do breed true, why wouldn't those who would like a smaller statured herd use Woodmagic genetics instead of the chondrodysplasia gene? Does the chondrodysplasia gene have advantages over the Woodmagic genetics that outweigh the disadvantages of chondrodysplasia?

[Broomcroft]

I mentioned that my first short bull produced tiny and large offspring, and my non-short bull produced far more consistently sized calves. It has been said in response somewhere that non-shorts breed more consistent offspring, size-wise, than shorts. But is it being said that really it's chondros carriers that produce inconsistent offspring, so presumably a short non-chondros will produce more consistently sized calves, possibly as consistent as a non-short?

I'll be looking for a new bull at some time and want consistent sized calves as far as it is possible with Dexters because most will go to the butcher.

[wagra dexters]

Clive, that goes without saying, the shorter non-chondro is equally as consistent as a taller non-chondro. But the bull is half the equation, no matter how pre-potent, and only after the female herd is of a fairly consistent, chondro-free line, can you expect to get much less discrepancy in the calf crop.
Margaret.

[Broomcroft]

Thanks Margaret. That's very clear.