Large heart"X Factor" Stallions

[Tonno100]

From the majority of your posts one could get the idea that you somehow feel that you know a little something about genetics in general and horses in particular. With that said, how can you be so sure that the gene or genes that determine heart size in the horse are not located on one chromosome. I am not saying that I agree with the theory, and I am not saying that all of Haun's assumptions are correct, but to say that it is "ludicrously unlikely that the genes for such a heart are on one chromosome" is not based on fact and is your assumption.

Probably a fair analysis, I clearly am taking an extreme line on the matter. That said, I find the idea that there is one gene that codes for "bigness" of the heart completely unpalatable.

One thing I am sure of is that the genes carried on the X chromosome have a major impact on racing ability, if not, how is it that certain female families continue to produce superior runners generation after generation when in fact the only genes they continue to share undiluted are the genes carried on the X chromosome?

They also produce plenty of rubbish runners generation on generation, just less than the average family due to their genetic strength overall. They also don't carry the X chromosome genes undiluted, take Henrythenavigator for example, his X chromosome could have been the one Sequoyah got from Brigid, but could also be the one that Sequoyah got from Sadler's Wells. I'd have given you that mitochondrial DNA is passed down the family line though, an area in need of further research and far more plausible than Haun's x-factor theory.

Personally, I think a superior family produces superior runners simply because they're drawing from a genetic pool that contains fewer weak links, therefore the resultant offspring in turn have, overall, a reduced chance of any weaker links (a simplistic view). The differences in athletic ability of an average runner and a top-class runner are not that far removed, every fraction of a second matters in this sport, and having strong genes in the female line to fall back on helps those horses rise to the top.

[DDT]

Tonno100

I agree that each foal could get a different X chromosome, what I said was the genes contained on the X chromosome are undiluted. For the sake of argument let's say Henrythenavigator received Special's X chromosome from his dam, the genes on that X chromosome are undiluted and are the same that were present in Special. Now, because Henrythenavigator is a colt, he only has one X chromosome, the genes carried on that chromosome are the only ones present and therefore will be expressed.

DDT

[xfactor fan]

Stallion owners have a HUGE interest in discrediting the X factor theory.

Let's play pretend for a moment, and there is some validity to the idea that the large heart gene is governed by something on the equine X chromosome.

As a stallion manager you have a successful race horse, Mr Superstud, with a larger than normal heart. ALL of the colts sired by Mr Superstud are not going to have his heart. If the greater cardio played a part in the sire's success, his sons are not going to live up to expectations. Mr Superstud will have overall better daughters than his sons but be a better broodmare sire.

What is going to the the effect on Mr Superstud's value as a stallion if it gets out that his colts are disappointing? The price the stallion manager can charge for stud fees? The hype that goes along with being a "sire of sires" ?

Back to reality, Haun is a sports writer, and while she brought the theory that the Australians had been researching to the public, she is not a geneticist, nor a scientist. Her economic interest is in selling books. Books sold=Money in pocket.

Books sell because of marketing. It is easier to sell dreams than to sell reality. Both of the X factor books concentrate on the dream that you too can breed the next Secretariat, if only you have the correct X.

If you go back to the original Australian study, they found that the heart size measured by EKG (the Heart Score) is larger in stakes horses than in claimers. Leading the Australian to believe that there is a positive connection between having a large Heart Score and racing performance.

There was another study done by the Danes ( or Belgians) on trotters that came up with the same kind of results.

And there are some human studies that indicate the same thing, in some sports having more efficient cardio is a benefit.


This all is pretty logical, extra cardio = extra performance.

Back to the Australians, perhaps inspired by the 14 lb pickled heart of Phar Lap, they started looking a where this came from. Their conclusion was that it is a sex linked factor, transmitted via the female line of the pedigree.

Which is where the whole X factor book theory came from.

[Tonno100]

Tonno100

I agree that each foal could get a different X chromosome, what I said was the genes contained on the X chromosome are undiluted. For the sake of argument let's say Henrythenavigator received Special's X chromosome from his dam, the genes on that X chromosome are undiluted and are the same that were present in Special. Now, because Henrythenavigator is a colt, he only has one X chromosome, the genes carried on that chromosome are the only ones present and therefore will be expressed.

DDT

I see what you're saying, but the "for the sake of argument" part is a whopping clause! I stand by my original assertion that the x-factor theory is simply a result of a search for a simple answer to an impossibly complex problem.

Also, I don't know what you're getting at by saying the genes are undiluted, you say this, then point to the fact that Henrythenavigator has to express his x chromosome. This is true, but the pool of x chromosomes that he could have got his from is just as diluted as the pool he drew the rest of his genes from.

Something else that might be relavent, though correct me if I'm wrong as it's been a couple of years since I studied this, but isn't one of the x chromosomes in females randomly de-activated and becomes a Barr body?

With regards to the greater heart size in stakes vs claiming horses, this may well by the case, but there's little lip service given to cause and effect here. Does the horses heart get to that size because it's genetically predisposed to doing so, or is it that big because the rest of the horse's genetics allow it to train to a level that allows increased cardiac size?

[going4stamina]

If you don't think certain genetic traits are only passed along the X chromosome, I suggest you study the impact of Girls and Women with Turner Syndrome (XO). Not only does the X have a major impact, but lack of one X can cause certain genes to not "turn on".

Height, heart, social skills, facial features, hairlines, and a myriad of other issues because of one missing X. The X is a very powerful dynamic, the most important dynamic for Mammals, perhaps.

[Linda_d]

If you don't think certain genetic traits are only passed along the X chromosome, I suggest you study the impact of Girls and Women with Turner Syndrome (XO). Not only does the X have a major impact, but lack of one X can cause certain genes to not "turn on".

Height, heart, social skills, facial features, hairlines, and a myriad of other issues because of one missing X. The X is a very powerful dynamic, the most important dynamic for Mammals, perhaps.

I don't think that's what's being argued, at least not by me. I just find Haun's theory -- a single gene determining heart size and largely determining racing ability -- implausible because of its simplistic nature.

[DDT]

Tonno100

The "for the sake of argument" was to pinpoint a name to the X chromosome for Henrythenavigator, it could be any of the many that could have been passed down the fact remains that he received his from his dam, not his sire, and all of the X chromosomes in the mix had their own genes, undiluted, in tact, when the eggs were created.

The fact that one of the X chromosomes in females is ramdomly de-activated has little to do with our discussion, but it certainly might have something to do with expression or non-expression of a large heart in fillies and mares.

There is a significant difference in mitosis (cells multiply) and meiosis (creation of gametes, eggs and sperm). Meisos is a very important event in sexually reproducing species, for two reasons, it maintains the chromosome constitution characteristic of the species through the generations by reducing the chromosome number in the gametes, and it brings about the final mixing of the parental hereditary material by the exchange of parts between homologous chromosomes. The X and Y chromosomes are not homologous and therefore do not exchange parts (genes) during either phase. This is why I said the genes on the X chromosome are undiluted.

DDT

[griff]

Have you guys also determined that the gene for muman baldness is also no longer carried on the X chromosome?/

griff

[Tonno100]

Tonno100

The "for the sake of argument" was to pinpoint a name to the X chromosome for Henrythenavigator, it could be any of the many that could have been passed down the fact remains that he received his from his dam, not his sire, and all of the X chromosomes in the mix had their own genes, undiluted, in tact, when the eggs were created.

The fact that one of the X chromosomes in females is ramdomly de-activated has little to do with our discussion, but it certainly might have something to do with expression or non-expression of a large heart in fillies and mares.

There is a significant difference in mitosis (cells multiply) and meiosis (creation of gametes, eggs and sperm). Meisos is a very important event in sexually reproducing species, for two reasons, it maintains the chromosome constitution characteristic of the species through the generations by reducing the chromosome number in the gametes, and it brings about the final mixing of the parental hereditary material by the exchange of parts between homologous chromosomes. The X and Y chromosomes are not homologous and therefore do not exchange parts (genes) during either phase. This is why I said the genes on the X chromosome are undiluted.

DDT

My point about your first para is that I agree with you, he did get his x from his dam, but not necessarily from his 'family'.

I thought you might be referring to chiasmata, but this does of course occur in the females, where there are two x chromosomes, so the x coming from the female is certainly recombinant to a certain extent, or diluted, if that's what you mean.

Been a while since my brain had this kind of workout!

[xfactor fan]

Heart efficiency can be enhanced by training. Don't know if the actual heart size increases.

Claimers verses Stakes horses. Wouldn't claimers and stakes horses be getting roughly the same amount of training/conditioning?

It might be interesting to look at horses before they start training, and horses after training to determine if, or how much the heart scores change.

Lets turn the argument upside down. Given that there is a large range in healthy heart sizes in Thoroughbreds, Arabians, Quarter Horses and Standardbreds, where does it come from, and how is it inherited?

As for X inheritance, there is a 50 / 50 chance of any given colt getting a specific X.

The argument for Special would be that she passed her X to her daughter Fairy Bridge, who gave it in turn to her son Sadler's Wells.

Sadler's Wells would have in turn given his X to Sequoyah who gave it to Henrythenavigator.

So if Henrythenavigator has the Special X coming via Sadler's Wells, there should be some similarities between Henrythenavigator and Sadler's Wells because they carry the same X.

You know that Sequoyah has one copy of the Sadler's Wells X, and one unknown X. Which could have come from Irish River, Raise a Native, Bold Lad, or Better Self.






Things that have been proven to lurk in the human X chromosome include Male Pattern Baldness and Hemophilia are perhaps the best know of the X linked mutations.

[DDT]

Toono100

You are correct that during the chiasma formation phase, in females the two X chromosomes do pair up and exchange parts at one or at several points, so the resultant egg contains an X chromosome that is diluted with genes carried on the X chromosome donated by the sire and the X chromosome donated by the dam. The result is an egg that contains an X chromosome that is genetically different, even if only by one gene, than the X chromosome donated by either parent.

You are right, this is a brain buster.

DDT

[tjobert]

I've read that Sham also had a large heart, but a diseased one. Assuming the X-factor theory relating heritable (healthy) heart size to performance is valid, is there a sub-set of inherited large hearts that are UNhealthy based on genetic predisposition, not damage? Also, I wonder if human hearts are in an evolutionary 'sweet spot' where increasing the size overwhelms the rest of the pulminary infrastructure, but thoroughbreds still have capacity in their infrastructure.

[Nessa]

I've read that Sham also had a large heart, but a diseased one. Assuming the X-factor theory relating heritable (healthy) heart size to performance is valid, is there a sub-set of inherited large hearts that are UNhealthy based on genetic predisposition, not damage? Also, I wonder if human hearts are in an evolutionary 'sweet spot' where increasing the size overwhelms the rest of the pulminary infrastructure, but thoroughbreds still have capacity in their infrastructure.

Sham's heart was found to be large and diseased when he died, but that doesn't mean that it was enlarged just because of disease. It could be that it was his heart was larger and perfectly healthy when he was younger and racing. You are probably correct that if a healthy heart can be inherited then there could be a sub-set of genes that can be responsible for genetically unstable heart structure. Perhaps this is why some horses have aortic ruptures and heart arrhythmias and collapse for seemingly no reason.
Many superior athletes have been found to have enlarged hearts, with incidents of disease, upon death. Maybe superior athletic ability can bring on heart disease later in life because the atlete no longer keeps up as good physical condition.

[aethervox]

I personally think that it's possible there's a gene on the equine X-chromosome that allows a larger than normal heart. I also believe it when Haun says that they've traced the gene through generations and discovered striking similarities in the ECG traces among families.

Having said that, I must also say that while the large heart may be an important factor to a horse's success, a horse also has to have the lungs, legs/conformation, and will to win to become a champion. I think Secretariat had all four of those things (I remember that they had to special order girths for him because the largest ones available commercially were too small). I also remember a documentary on PBS where a biomechanics expert analyzed Secretariat's gallop and said it was very efficient.

A difference in one gene can have drastic results. An example is the gene known as MSTN (or GDF8) which is on chromosome 2 and has been found in several species (including humans). if the MSTN gene doesn't work properly, there is muscle overgrowth (double muscling) due to an increase in both the number and size of muscle fibers. It is a recessive gene, so a person has to have 2 mutated copies to be affected. Other than the muscle size, the animals and people affected by this gene seem normal.

There's also the case of Hemophilia, where a mutation in one gene on the X chromosome (either F8 or F9) causes the body to produce abnormal coagulation factors (VIII or IX), meaning the blood doesn't clot as fast as in normal people. Because it's an X-linked recessive gene, women are only fully affected if they have two copies, while men with the mutated gene are always affected.

There's a really good website on genetics at http://www.genetics.com.au/

[xfactor fan]

Working backwards, it is true that TB's have a large range of heart sizes. From huge, 22 lbs of Secretariat, to almost as huge Sham, Key to the Mint, around 18 lbs, down to 8lbs or so.

Now what causes the range?

Cross overs, are when the same chromosomes exchange body parts so a segment of Chromosome 6, from the maternal line gets swapped for a segment of the paternal Chromosome 6. Resulting in the creation of two brand new chromosomes, one that is mostly maternal with a bit of paternal, and one that is mostly paternal with a bit of maternal.

Now sometimes this swapping is not quite equal, and if the gene sequence is A-B-C-D-E-F, one copy may get A-B-B-C-D-E-F and one copy will get A-C-D-E-F. The extra copy may be fatal, or may cause an extra something. Same with the bit with the deleted gene.


In humans there is a "thrill seeking" gene. Population with a high frequency of low levels of this gene tend to be risk adverse. Within this population, individuals with high numbers of this gene would be prone to high risk activities. And in fact this is the situation in Japan, where most of the population has low numbers of copies of this gene, and are pretty risk adverse. Japanese who like to engage in high risk activities tend to have multiple copies of the gene. I think the example from the study was a Japanese skydiver.

On a biochemical level this may be related to fear/flight reactions. And the difference between those who feel ill after a shock, and those who get high after the same shock. And on a historical level the population needs both types in the population.


This type of mis-copying is what I think is going on with hearts. Some sort of extra coding for heart size, and if for example one copy of H results in a 1 lb heart, and the average horse has H-H-H-H-H-H-H-H (8) then there could be a population with H-H-H-H-H-H (6) and H-H-H-H-H-H-H-H-H-H (10) giving a range of heart size from 6 lbs to 10 lbs. Which is pretty much what we see in the equine population.

And of course this is all speculation, and it could be an entirely different process going on. But to date I haven't seen anyone coming up with a different theory.