X-factor Question

[skennedy]

Hello Everybody,

I'm new here and I apologize if this question has been asked before. I'm interested in the pedigree of a horse named Epic Win, specifically whether he could have had the large heart gene.

In his pedigree

http://www.pedigreequery.com/epic+win

...the x-factor trace seems to stop after Whirlpool. Does this mean that Furwin definitely did not have the gene, or that there is no evidence to say for sure that she did have it? Similarly, how do they know that Whirlpool was a single copy mare? Is it just that there is no evidence to say for sure that she had a double copy?

Thanks for your help, and any other comments you may have about the pedigree are welcome. I find it very interesting to read the discussions you all have on here, even if I only understand a little of it.

Steve

[Pan Zareta]

It just means that Furwin has not been designated an x-factor mare in the db here. I've no idea if she was ever heart-scored (u/s & ECG) and she was unraced, but on the strength of progeny performance (Epic Win's) Furwin has now been designated an x-factor mare.

[skennedy]

Thank you, PZ.

So the strength of Epic Win's performance indicates that both he and his dam are x-factor carriers. Does this mean that all of Epic Win's daughters will now show up as x-factor mares?

And that in turn answers my unasked question about the x-factor data in the database. Some of it is "hard" data from EKG and ultrasound, and some of it is pure speculation based on progeny performance.

Thanks again. Happy Mothers Day to all the Moms and Dams out there.

[Pan Zareta]

So the strength of Epic Win's performance indicates that both he and his dam are x-factor carriers.

It suggests they are, so I gave them the benefit of the doubt and designated Epic Win & Furwin X-factor carriers.

Does this mean that all of Epic Win's daughters will now show up as x-factor mares?

I *think* so.

And that in turn answers my unasked question about the x-factor data in the database. Some of it is "hard" data from EKG and ultrasound, and some of it is pure speculation based on progeny performance.

Correct. Hard data or direct observation is the only way to be sure. Individual or progeny performance is not always a reliable benchmark. Some mediocre performers and producers are known to have a large heart, and some individuals with a normal heart were stand out performers on the track. Bold Ruler would be a notable example of the latter.

Thanks again. Happy Mothers Day to all the Moms and Dams out there.

On behalf of my my mares and myself, thank you.

[tbrace]

However,
If a mare has the x gene and the sire's dam has the x gene, then genetically, the off-spring is a very high probability of having the x gene.

It doesn't mean that the off-spring WILL HAVE a big heart, physically. That can only be measured post mortem or by X-ray/ultrasound.

[xfactor fan]

In theory the X factor is on the equine X chromsome.

The sire with a X factor dam has a 50% chance of getting the large heart X from his dam.

Same with the dam. If the dam has the large heart X she has a 50% chance of passing it on to her foal.

Colts have no chance of getting the large heart X from their sire, as they get his Y, and of course are colts. The filly's get their sire's X, and he's either got it or he hasn't. If he doesn't have it, the fillies don't have it either.
IF he does have it, they all will get the large heart X from their sire.

The dam can pass on either the large heart X, or the normal heart x so the possible combinations would be:

XX, Xx Xx or xx.

Then due to X chromosome inactivation where only one X is expressed,
the XX, will show up with a large heart, the xx will show up with a small heart, and the two cases of large and small heart x chromosome, will have a random chance of either x being expressed.

And just to complicated matters which x will be expressed may not be exactly random.

If as much of 25% of the performance of a colt is due to the heart size, then there is going to be a guaranteed drop in performance of 25% of colts sired by a large heart stallion. (Which is something that stallion owners are not motivated to talk about, and may explain why stallion owners don't think too highly of this theory)
The fillies are going to be better than the colts, as roughly half of them will express the large heart.

The best cross is a small heart stallion with a good record--low cardio, but everything else in top form, and a large heart mare--with luck one with a good race record too.

Hope that the genetic dice roll the combination of the Y , and all the other top flight genetic stuff from the stallion, and the large heart X and good stuff from the mare.

And of course you could get a small heart x from the stallion and a small heart x from the mare, or a large heart X from the mare and due to X chromosome inactivation this X is not expressed, or a small heart x from the stallion and a large heart X from the mare.

And I'd be very cautious about believing the X factor status of any horse of any sex without testing that horse directly.

[skennedy]

Hi xfactor fan. I was hoping you'd chime in here.

I understand the basics of x-factor inheritance. The new wrinkle for me is the part about mares with 2 x-factor genes only expressing one of them because one X chromosome is inactivated. That must have come out since I had genetics, back in the Dark Ages. Does this inactivation happen with all chromosomes or is it unique to the X?

I have a young horse coming on trial next week. He's out of a daughter of Epic Win, so he has a 50% chance of having the large heart and he comes from Pleasant Colony on the top side. So it sounds like the kind of combination you would like.

I'll have him vetted next week. Will most vets know what I'm talking about if I ask for a heart score? And I'm thinking an ultrasound would be better than an EKG, yes?

Good to talk to you,

Steve

PS. It's been over 10 years since Haun published her first book. I'm surprised nobody has identified the gene or developed a genetic test yet. Surely someone is working on that, aren't they?

[Pan Zareta]

I understand the basics of x-factor inheritance. The new wrinkle for me is the part about mares with 2 x-factor genes only expressing one of them because one X chromosome is inactivated. That must have come out since I had genetics, back in the Dark Ages. Does this inactivation happen with all chromosomes or is it unique to the X?

It's unique to the X. All females are essentially a ~50/50 mosaic of the maternal and paternal X chromosomes. Early in embryonic development each cell randomly deactivates one of its X chromosomes. This occurs at a multi-cellular stage. Otherwise, far more human females would be color-blind or have hemophilia. The same X will be deactivated in all subsequent divisions of those cells. In cats coat color is on the X chromosome, and the classic visual example of random X-inactivation is the calico cat.

It's been over 10 years since Haun published her first book. I'm surprised nobody has identified the gene or developed a genetic test yet. Surely someone is working on that, aren't they?

The entire equine genome has been sequenced, including the X chromosome. None of the genes identified on the equine X offers any obvious proof for Haun's hypothesis, but the specific function of some of them has yet to be determined.

[skennedy]

Hi PZ, I sent you an email. Thanks for the info about X inactivation, and interesting that the equine genome has been sequenced.

I remember reading something about a geneticist at the University of Kentucky that was working with Haun on identifying the X-factor gene.

I'm no expert in this area, but I was under the impression that an RFLP test was fairly straightforward to develop if you had access to known positive and known negative blood samples. (RFLP stands for Restriction Fragment Length Polymorphism, I think. And I think that's the kind of test they use in paternity suits.)

I guess if it was that easy it would have been done by now.

[xfactor fan]

First ignore the top side of the pedigree because your colt got the Y from his sire, making him a colt. The only place the X can come from is the dam side.

The X inactivation is something that Huan didn't understand in the first book, and got wrong in the second book.
To expand a bit on Pan Zareta's explanation.

Males have one Y (making them a male) and one X. Females have two X's. Apparently, only one X is needed. So sometime between fertilization and implantation--so this is very early--the two X's flip a coin and one retires from the field. Curls up and migrates to the membrane between the nucleus and cytoplasm. If your class covered Barr Bodies--that drumstick shaped body is the extra X chromosome.

Look at how the color falls on a calico cat, large random patches of orange or black fur. Calico are all female, and in the orange patch the X that has been turned off is the X with the gene for black coat color.

So a mare could have nerves and tendons in each from a different X cell line.

Questions that remain unanswered:

Is there a gene, or gene complex that resides on the X that controls heart size and function.

Is the heart function in females a mosaic? Or does it derive from only one cell line? I've not seen any data on this issue.

Please take under advisement that the X factor books were written to sell books. Not to do good science.

As for the actual X's.

Look at the dam sires:

Flying Fury 50% chance of getting this X
Menow 25% chance of getting this X
Sir Gallahad 12.5% chance of getting this X
Sardanapale 6.25% chance of getting this X

On a practical side you have to go back to Scylla to find any runners. So if the database info is accurate, that means that you have to go back to the third dam before finding horses with racing ability. Argues that none of the first three dams have an expressed large heart X.

If this from a racing family, or are the horses from jumping or eventing lines?

Hope this helps

[skennedy]

First ignore the top side of the pedigree because your colt got the Y from his sire, making him a colt. The only place the X can come from is the dam side.

I know, I only mentioned the sire because of your comment about the ideal cross being good blood and speed and conformation on the top and an X-factor mare on the bottom.

Questions that remain unanswered:

Is there a gene, or gene complex that resides on the X that controls heart size and function.

Is Haun's basic premise of an X-linked, inheritable, larger-than-normal heart in doubt? The circumstantial evidence seems good.

Please take under advisement that the X factor books were written to sell books. Not to do good science.

Agreed, I didn't read the books, but the things I've read on the web all fall into the category of what scientists disparagingly call "Sunday Supplement Science". Interesting reading, but lacking any scientific rigor.

As for the actual X's.

Look at the dam sires:

Flying Fury 50% chance of getting this X
Menow 25% chance of getting this X
Sir Gallahad 12.5% chance of getting this X
Sardanapale 6.25% chance of getting this X

PZ and I have already jumped to the conclusion that Whirlpool passed on an X-factor heart to Epic Win. Just because she was in the database as an x-factor mare. But (assuming he had a large heart) it could have come from Flying Fury if he got it handed down through 4 generations from Sweep. And Whirlpool could have been a double copy mare too, I guess.

I was trying to figure out which x-factor line Whirlpool would have had. Haun talks about the Princequillo heart, and the Mahmoud, War Admiral and Blue Larkspur hearts also. But I don't know enough about the various large heart horses to be able to pick it out. St Simon looks like a likely source, do you know which of the above inherited St Simons heart?

Is this from a racing family, or are the horses from jumping or eventing lines?

Hope this helps

Eventers, and yes it does, thank you very much.

I still have the question about the vet exam. Will they know what to look for and measure? I saw a recent publication about using ultrasound to measure heart size in horses (someone posted it on here, I think) but I haven't read it yet. Eventing horses are bred for endurance just like TBs, so I imagine the large heart gene is present in the great ones, but few people in the eventing world have heard about it.

I'm afraid the vet is going to look at me and say "Huh?" Is ultrasound the best measurement, and what do I tell the vet to look for? I guess I'd better read that paper. Thanks, Steve.

[Pan Zareta]

I remember reading something about a geneticist at the University of Kentucky that was working with Haun on identifying the X-factor gene.

I'm no expert in this area, but I was under the impression that an RFLP test was fairly straightforward to develop if you had access to known positive and known negative blood samples. (RFLP stands for Restriction Fragment Length Polymorphism, I think. And I think that's the kind of test they use in paternity suits.)

Haun ran her hypothesis that the large heart is a sex-linked dominant trait by several professionals, including equine geneticist Dr. Gus Cothran, then of UKy now of TAMU. (Since specific information re. transmission and expression of the trait has would undoubtedly have considerable commercial value I'd be very surprised if it wasn't still under study somewhere.)

RFLP is one of several methods that can be used to examine DNA, but it doesn't actually sequence all or part of it. Crudely put, it's based upon the fact that there are hereditary differences between individuals in where a particular strand of DNA will cleave.

I still have the question about the vet exam.

Any AAEP vet should be able to assess heart size through echocardiogram (u/s) and appreciate why you'd be interested in having that information.

Is the heart function in females a mosaic? Or does it derive from only one cell line? I've not seen any data on this issue.

I haven't either. It's tempting to extrapolate that it's mosaic from the fact that hemophilia and sex-linked color-blindness only occur in human females if they're homozygous for either trait (both transmitted as sex-linked recessive). This suggests that multiple embryonic cell lines are present during formation of the highly specialized tissue that produces the color-perceiving cones on the retina and that which produces (clotting) Factor VIII. Further substantiating the involvement of multiple lines is the fact that even though their coagulation time or color-perception are within normal limits females heterozygous for hemophilia or sex-linked color-blindness seldom have the same level of circulating Factor VIII or the same complement of cones on the retina as do females homozygous for the normal alleles.

However, it apparently hasn't been ruled out that some unknown factor or mechanism is selectively reactivating some or all inactive X's when it receives atypical 'instructions' from an active X carrying the alleles for hemophilia or sex-linked color-blindness (or one of several other sex-linked recessive diseases).

[xfactor fan]

It is so good to have someone who can talk reasonably about this:

Argument for a single source of the heart tissue is that the heart is a very early organ to form, and the earlier it forms the more likely that the third cell to the left in cell division 4 of the embryo gets tapped to be the foundation of the cell line that becomes a heart. (I have this vision of a guy with a clip board handing out assignments--you there, go form a heart--and you over there make me some liver. And you, don't hide, get to be a gut. Don't whine about it just go make some intestine cells. ) Well, maybe there isn't a guy with a clip board, but something tells the cells change from blob to structure.

So little is still known about the organization and differentiation of the early embryo that who knows what exactly is going on. Some of the human research--mostly not published--on human in vitro fertilization suggests that the more the egg and sperm is "messed" with the higher the chance of all sorts of unpleasant side effects.

I suspect it is going to take another generation to create the tools before some of these question can be answered.

[Pan Zareta]

Argument for a single source of the heart tissue is that the heart is a very early organ to form, and the earlier it forms the more likely that the third cell to the left in cell division 4 of the embryo gets tapped to be the foundation of the cell line that becomes a heart. (I have this vision of a guy with a clip board handing out assignments--you there, go form a heart--and you over there make me some liver. And you, don't hide, get to be a gut. Don't whine about it just go make some intestine cells. ) Well, maybe there isn't a guy with a clip board, but something tells the cells change from blob to structure.

LOL! But I'm a bit confused re. "third cell to the left in cell division 4". After cell division 4 there should be 16 cells. That's morula (blob) stage. Which way is left? Sources vary as to how far along in cell division the embryo is when random X-inactivation takes place. The usual 'guestimate' seems to be at the 32 cell stage. Doesn't the preferential inactivation of the paternal X in the cell(s?) destined to become the extra-embryonic tissue take place before this? Does pluripotency terminate in all cells at the same time?
(Is it obvious enough that I've had no formal instruction in embryology in a lo-ong time? )

[skennedy]

LOL! But I'm a bit confused re. "third cell to the left in cell division 4".

I took the meaning to be roughly "some cell in the early-stage embryo". Chosen by the guy with the clipboard.

I think there would need to be a single cell source for the developing heart. If it were truly a mosaic, you could have one group of cells developing into normal sized ventricles, say, while another line of cells tried to develop double or triple-sized atria. Seems like that would be problematic. But if it were finer-grained than that, maybe you could end up with a 1.5x heart. As if half the heart cells tried to grow a large heart and the other half grew normally.

I remember reading somewhere that the distribution of heart sizes does not seem to be biphasic as Haun's theory would predict. But more of a continuous bell-curve. I really wish I could see that data.

Doesn't the preferential inactivation of the paternal X in the cell(s?) destined to become the extra-embryonic tissue take place before this?

So the X-inactivation is not truly random, but is slanted so that the paternal X is more often inactivated... And how does the cell know which is the paternal X - it was a maternal X in the sire after all.

Does pluripotency terminate in all cells at the same time?

It must not, because I know that fully developed adults have at least one kind of pluripotent cell (mesenchymal cells) for things like bone repair.