Tobiano Suppression - A Study

[Silverine]

So after a few mini experiments I had the question: are there tobiano suppression genes in the game?

So far we (the color community) have "figured out" that the body and leg markings are apparently controlled separately. From my previous study it seemed like spreading white onto the body was controlled by a recessive switch gene - you needed two copies of it for the horse to express white above the legs. (Not including minor facial markings like stars, strips, and snips.)

Now I'm wondering if we have instead been dealing with dominant suppression genes. Which I admit at face value seems like the same thing but is treated differently in my mind. These only came to my mind because white suppression genes do exist in real life. Not all of them are known and there are an enormous variety of them, accounting for the large differences in white patterns of the same type in real-world horses.

The horses that made me most curious about the possibility of suppression genes on HWO are the AC Przewalskis. No matter who I bred to them or how much white the non-Prze parent had, the offspring never expressed any white what-so-ever. (Please, if you have foals from an AC Prze parent that show white markings of any kind, show them to me! I would love to see some if they exist.)

Here are the foals I've kept from these experiments, and their parents:
The Sire

Foal 1	Mother

Foal 2	Mother

Foal 3	Mother

Foal 4	Mother

The mothers of the foals in question are from my full-white line and should, theoretically, be homozygous for tobiano. It is exceedingly unlikely that all three of the mares (the first two foals are from the same mare) are heterozygous. Even if they did all happen to be heterozygous it would be incredibly unlikely that all four foals would not have inherited tobiano. Still, it is theoretically possible.

However, I believe that there is something about the Przewalski (and possibly other AC breeds, but I haven't looked into those) that suppresses the tobiano expression. This horse is why I think that:



He looks like a normal bay dun with minimal tobiano, right?

Well here are his parents:





Not a lick of tobiano white to be seen on either of them. So where did their colt get his tobiano?

Both of the colt's parents are descended from AC stock. His sire is an unmarked Caspian crossed with a sock-marked Forest Horse. The dam is by a Przewalski out of a fairly loud Shetland mare. With this heritage as I have stated it I see two possibilities of what happened:

1. The most likely scenario is that the sire (Caspian x Forest Horse) received a To switch from his mother (Forest Horse) but did not receive any white patterning genes and so did not show any white. The mother (Prze x Shetland) did not receive a To switch from her mother (Shetland) but did inherit white patterning genes. When the two were crossed the sire threw the To switch and the dam threw the patterning genes.

2. Less likely, but still possible - one or both of the parents is a suppressed tobiano. If this is the case, unlikely as it is, it means that one or both parents carry both the tobiano switch and white patterning genes as well as a suppression gene that prevents tobiano expression.

There is a caveat to the entire scenario above. I recently discovered an expression of tobiano so minimal that I at first thought it was dust on my monitor.

Check out the star on this mare (if you can find it):


Here is the mare in question in full:



She quite clearly has other tobiano markings or I would have thought her "star" simply a rendering artifact (if I noticed it at all). In fact, I'm still not sure it's not a rendering artifact. But, if it is tobiano then it is possible the dam of the above "mystery" tobiano had a star such as this and I missed it completely. It would have blended in well with her light coloring and now that she's grown her mane is too long for me to be able to check. I don't see anything in her younger gallery pictures, but they are all full-body shots and do not show her forehead in enough definition for me to be completely certain.

So, what does this all mean for tobiano suppression, if that is something that actually exists in the game?

From what I have seen and experienced, I believe there are 3 different "suppression genes".

• Type A - Seen in Przewalski Horses (and potentially other AC breeds, though that is untested). This gene completely prevents tobiano expression regardless of the presence of a To switch gene and white patterning genes. This gene is dominant.
• Type B - Seen in Belgians, Forest Horses, Tarpans, and Turkmenes. Allows for "slipped" tobianos - horses that display socks, stockings, and/or facial markings, but no white on the body. This gene is recessive to Type A and dominant to Type B.
• Type C - Seen only in North African Barbs and Shetlands (that I am aware of). Allows for full tobiano expression. This gene is recessive.

At this point I have not done enough testing to determine anything with absolute certainty, but I thought my theory fleshed out enough to share it with the community and open it up for input.

My next step is to age the Prze x full-white foals and breed them together to see what they produce. I also have vague plans to (at some point) purchase mares of many different breeds from the AC and breed them to my full-white studs to see what those crosses produce. Until then I'm afraid there are no definitive conclusions.

So, fellow HWO players, what do you think? Do you have any horses that you think might be "afflicted" with a suppression gene? Do you have any "mystery" tobiano foals from un-marked parents? Do you have any AC Przewalski offspring with tobiano markings?! Let me know!

[Silverine]

Here's the post I told you about.

I have no idea if I'm completely off base or not. I won't ask for answers, but I will ask that you let me know if I'm completely off my rocker. Also would love to know if that little "star" is actually a star.

[BlackOak2]

Thank you! Adding it to my quick-links.

[Cypress Creek Elites]

Thank you! Adding it to my quick-links.

Another thing I've noticed (from my brief Baladi project) is that AC Arab x AC NAB will never throw a foal with body tobiano, regardless of whether the NAB was het or homo tobiano, and regardless of how widespread the markings were.
But the descendants of the first gen Baladis eventually got full body markings.
It was weird.

[Silverine]

Another thing I've noticed (from my brief Baladi project) is that AC Arab x AC NAB will never throw a foal with body tobiano, regardless of whether the NAB was het or homo tobiano, and regardless of how widespread the markings were.
But the descendants of the first gen Baladis eventually got full body markings.
It was weird.

That would seem to me to be another point in favor of some breeds have genes that suppress tobiano. I haven't worked with AC Arabs at all so I have no experience with what they throw.

Now I really need to clear some space somewhere and gather up a bunch of AC mares to see what I can pop.

[Silverine]

Sadly I rehomed a foal before realizing it was useful for looking into tobiano suppression. Here is a saved photo of the foal in question:



Neither of the foal's parents have visible tobiano.
The sire is a brown pangare Belgian x Forest Horse:




And the mother is one of the Prze x full-white Gypsy mares:




The high white stocking on the foal leads me to believe that the dam has extensive white genes from her dam that are being suppressed. She should be heterozygous Type A suppression (one copy of Type A, one copy of Type C). The sire is descended from two breeds that display socks and stars so I would guess that he is homozygous Type B. So I am guessing that the dam passed on the To switch, white genes, and Type C suppression while the sire passed Type B suppression, leading to the foal with short socks and one loud stocking.

When looking back through the dam line's white expression, a lack of white on the face and the hind legs is not unusual. Those areas were the last to be fully covered in that line's progression. And of course body white would not be possible with Type B suppression.

[Silverine]

Just popped a full sibling to the rehomed foal from last night:



This one does not have quite as much white as the first but does still display tobiano. She has been retained to continue my experimentation.

Links to her parents for reference.

[BlackOak2]

Very interesting.

Surprising that both foals are so suppressed though. If both parents are hetro- for suppression genes, ...
Oh... reading back to see what you said, you did say that you suspected the sire might be homo- for suppression.



Okay, not so interesting that both foals are sockies only. Although with such little sockies at all, that is a bit interesting outcome.

[Silverine]

As I said with the first foal, legs (especially the hinds) and face were the very last parts to be finished in my Gypsy line. Their bodies and necks had full coverage quite a bit before I got those areas to be included as well. So it makes sense to me that their descendants would have less expression in those areas than over their bodies. However it will be quite interesting to see how much body white I can get from a direct cross of two Gypsy x Prze horses.

[Silverine]

Another foal from two unmarked Prze offspring:




This one is by one of the Prze x Gypsy colts and out of a Prze x Rhineland Heavy Draft mare. The RHD line only ever showed socks so I'm guessing that line is also lacking Type C and thus this foal is a heterozygous BC.

The parents:





The RHD grandmother:



The grandmother doesn't have any tobi herself, but her sire had a single stocking. Such limited markings were the only to ever appear in my RHDs.