Sorry...another X factor question

[Patuxet]

Over on the Paulick Report I was amused to find this comment appended to an article entitled: "RIP Sadler's Wells":

"He had a great, great heart that will be carried on through his descedents like Master of Hounds. Marianna Haun"

[xfactor fan]

Well, at least she's working within the theory. Sadler's Wells is the Broodmare sire of Master of Hounds, and so has a 50-50 chance of carrying the SW heart. (Whatever size it was)

[Linda_d]

My problem with the large heart theory is that it just seems too simplistic to me. When I think of all the genes needed to produce something easy and sort of one dimensional like color and markings -- a buckskin horse with three ragged stockings, a wide snip down his nose, and blue eyes (minimally black, agouti, cream, sabino, and splash) -- I find it hard to believe that heart size comes down to 1 gene.

I also find the "proofs" for this theory based entirely on assumptions of which long-deceased horses had large hearts and which don't to be the most specious kind of "science". It's a convenient explanation for what's easily observed in that some stallions produce better daughters than sons. However, that's been observed in other breeds besides TBs and not all of those breeds have been bred for racing. How is a large heart a performance advantage for a cutting horse or a show jumper? What about stallions that produce better sons than daughters or those that produce good ones of both sexes?

[xfactor fan]

Yep, the problem is that lots of assumptions are being made about dead horses.

However, there are a couple of things to note. The whole idea was started by some scientists in Australia. They were looking into their own home grown hero Phar Lap. His large pickled heart is on display.

Both Secretariat and Sham, had large hearts. Both were out of Princequillo mares.

A large heart horse that has a bad case of the slows, is still slow. A horse with a large heart that doesn't like racing, won't like racing, no matter what size the heart is.

Haun is a writer, her job depends on selling books. The whole TB industry is based on dreams of having the "Big Horse" , Her books speaks to that dream. If you just breed this way, you will get the next Secretariat. She's in the business of selling the dream, not doing good science.

I'd like to talk about dogs for a moment. Fact, small dogs live a lot longer than large dogs. And very large dogs have a very short lifespan. As little as 8-9 years in some of the large breeds (Great Danes, Irish Wolfhounds) In part this is due to the fact that dogs pretty much have the same size hearts. The a normal size heart isn't up to the strain of pumping all that blood around the huge body.

Human giants have the same problem, normal size organs, oversize body, and they tend to die young.

Some Greyhounds--one of the other breeds bred for racing-- have a lifespan that is shorter than small dogs, but longer than the other dogs of their size. If you google dog breed sites, many of them have life expectancy listed under breed features.

So there is clearly some mechanism working to select for larger heart size. And just as clearly it has to be under some kind of genetic control.

I'd love to see everyone that has a heart score post that number to the database, and see where the data goes.

And no, I don't think heart size is a simple dominant/recessive on one of the regular chromosomes. I'm not even sure it is a multi gene process ( like bay) . Humans have been breeding horses for a long time, and if it was a simple pattern, we humans would be better at it.

In terms of the theory:

Large heart stallions will have better daughters than sons, and live through their maternal grandsons. Sons will have the heart of their dams, some of which might have large hearts. And since having a large heart gives a racing "boost" they are actually (once the boost is taken away) not as good in all the other elements that make up a race horse.

And this pattern seems to fit Secretariat.

Normal heart size stallions will run to their breeding and sire to their breeding. No boost, but also no dip in the other genetic qualities that make up a racehorse.

Small heart size Stallions, if the are successful race horses, have a handicap to overcome. Everything else has to be in prime shape. These horses when bred will have a pattern of great sons, and not so great daughters. The sons get all the stallion's good genetic qualities, plus the normal to large heart size from their dams. Daughters will pass on the small heart, and if all the rest of the genetic components to make up a racehorse aren't in place, they won't be all that successful.

This pattern fits Northern Dancer.

Is this the way it works? Maybe, maybe not, but is does fit the observed patterns.

[Pan Zareta]

However over-simplified Haun's hypothesis seems now it should be remembered that various highly respected professionals in veterinary medicine and genetics believed it had merit and endorsed her publications. Her hypothesis was developed from the observation that the one thing common to the pedigrees of all horses known (from necropsies) to have had large hearts was descent in a line with no male repeats from Everlasting b f 1775 x Eclipse (other daus. of his have since been implicated). That no single, obvious, "large heart gene" has been found on the equine X chrom. doesn't mean there are no genes whatsoever there that influence heart size.

[Georgerz]

I just read in the Huffington Post that the "gay" gene is passed in the X chromosome in humans. The article explains how this impacts the female relatives of gay men.

http://www.huffingtonpost.com/2012/06/1 ... 90501.html

[Pan Zareta]

I just read in the Huffington Post that the "gay" gene is passed in the X chromosome in humans. The article explains how this impacts the female relatives of gay men.

My impression from that one article is that this is no more than a hypothesis from phenotypic evidence as was the equine "X factor" (sex-linked transmission of heart size) which, while plausible enough 15 years ago, is almost certainly wrong.

[xfactor fan]

I'd be concerned about the data. Stasticaly in a large family the younger siblins are more likely to be gay. So in a family with 4 boys, the youngest boy is more likely to be gay.

If it is X linked, then in a family of several boys all the boys with the same X would be gay, birth order shouldn't matter.

[Linda_d]

I just read in the Huffington Post that the "gay" gene is passed in the X chromosome in humans. The article explains how this impacts the female relatives of gay men.

My impression from that one article is that this is no more than a hypothesis from phenotypic evidence as was the equine "X factor" (sex-linked transmission of heart size) which, while plausible enough 15 years ago, is almost certainly wrong.

It seems to me that if there's a "gay gene" on the X chromosome, then the sisters and neices of gay men would be prone to having more sons who were gay. Gay men would then have more gay male cousins. Conversely, gay men ought to have gay maternal uncles and great uncles. In other words, there would be maternal families that tended to produce more gay males than average.

I suppose that one could argue that society has limited our ability to determine if that is true. However, you could also argue that maybe because the mothers and aunts of gay men had more children, they had better chances of producing gay males.

[stancaris]

Pan Zareta: The X factor theory according to you is almost certainly wrong.

My question: Does this mean in your estimation that there are no genes on the X whatsoever that may interact with autosomal genes to produce the large heart in racehorses? What I am driving at is multiple gene effects whereby genes on different chromosomes can produce the large heart. Perhaps for argument sake, a purely hypothetical idea that genes on the 6th chromosome and genes on the 17 chromosome along with genes on the X chromosome all working together produce the large heart. Of course this idea would still negate the simplicity of Haun's theory that it was a single gene on the X that causes the large heart phenotype.

Now lets say the large heart is actuallly due to the above multiple gene effects and that genes on the X are involved, could we now say that although Haun's simplistic theory is wrong she still has struck on an important hereditary characteristic that is at least in some way X related and that many of her conclusions about the transmission of the X would still be of interest to geneticists worldwide?

[Pan Zareta]

Pan Zareta: The X factor theory according to you is almost certainly wrong.

My question: Does this mean in your estimation that there are no genes on the X whatsoever that may interact with autosomal genes to produce the large heart in racehorses? What I am driving at is multiple gene effects whereby genes on different chromosomes can produce the large heart. Perhaps for argument sake, a purely hypothetical idea that genes on the 6th chromosome and genes on the 17 chromosome along with genes on the X chromosome all working together produce the large heart. Of course this idea would still negate the simplicity of Haun's theory that it was a single gene on the X that causes the large heart phenotype.

Now lets say the large heart is actuallly due to the above multiple gene effects and that genes on the X are involved, could we now say that although Haun's simplistic theory is wrong she still has struck on an important hereditary characteristic that is at least in some way X related and that many of her conclusions about the transmission of the X would still be of interest to geneticists worldwide?

Genome scans using the 75K SNP chip have yielded no evidence at all of ANY mediator for heart size on the X chromosome. This subject was discussed at length in The Mares in Great Sires thread.