Archive | October, 2011

More roaning and ticking

Dexter1

Another kind of roaning that is often attributed to the sabino gene is the kind seen on this chestnut tobiano pony, Dexter. This has a softer look than the “laced” edges that Dexter’s sabino-tobiano stablemate Splash has.

What makes Dexter unusual, though, is that he has a solid face.

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He does have a white patch on one side of his chin which does not reach up to his lower lip, which can just barely be seen in this picture. (Because it is really under his chin it is hard to get a good image.)

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It may be that modifiers are suppressing the sabino gene to such an extent that his chin patch is all that is left. It’s also possible that this type of roaning is itself some kind of modifier, and that the white on his chin is unrelated. The commonly accepted rule is that tobiano by itself does not create white on the face, though both myself and others have had reason to question the absolute nature of that rule.  (Because that statement is nigh upon heresy to many horse color enthusiasts, elaborating on that probably merits a separate blog post at a later date.)

But it is pretty clear that this is different from true roan. Here is what true roan, when combined with tobiano, looks like. (The photo comes from Reasontobecrazy stock photography.)

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Here is a close-up of another roan tobiano.

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Notice how the roaning is evenly distributed across the spots. Now compare that to a close-up of Dexter’s hip.

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It’s also different from the roan patches that are sometimes seen on tobianos, particularly homozygous tobianos like the one below. Those tend to be rather random, whereas the roaning on horses like Dexter are concentrated around the borders of the dark patches.

Here is a close-up of roughly the same area on Dexter.

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Here is another horse showing the same kind of softly roan edges,  although he has the white face markings that Dexter lacks. (The photo comes from Citron Vert Stock.)

tobiano_pony_by_citronvertstock-d3er0932

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Roaning and white ticking

Lazy5Belgian

Roan and sabino tend to be catch-all terms for horses that have white hairs or white markings. In the next few posts I wanted to share a few horses that a generation ago would simply have been called roan, but that have white hairs or ticking from something other than the true, dark-headed roan gene. The general convention now is to group horses like that in with the sabinos, but their actual relationship to the sabino patterns is not really known.

This American Belgian is a pretty typical roan-like pattern that is probably a type of sabino. Horses like this lack the very distinct dark points of a true roan, which are usually apparent even when roan gets combined with sabino markings.  His legs are dirty, which hide the fact that he does have white feet, but compare his legs to this true roan with flashy white markings:

The upside-down “V” formed by the dark points on his front legs, which is one of the hallmarks of the true roan pattern, is particularly visible in the video. This Belgian does not have those, even though his front socks do not extend far up his legs.

Notice, too, that the roaning not only extends into areas that would be dark on a true roan (like the face and lower legs), but it is quite uneven. His neck, for instance, shows very little roaning. Even more interesting are his hindquarters, which have patches of less intense roaning. For those artists familiar with etching to achieve roaning effects, it looks like the person doing his pattern got bored with the process by the time they reached his bum! These kinds of revertent patches are pretty common in sabino roans.

He also had an unusual roaned patch on the border of one side of his blaze.

I suspect that a lot of the Belgians (of American breeding at least) registered as chestnut roan in the last fifty years are in fact sabinos like this guy.

He isn’t that unusual, as sabino roans go. What is interesting is that it seems possible to get sabinos where there are fewer indicators that the pattern is there beyond the roaning. There are a number of instances where horses from sabino families have body roaning and a blaze, but no other white markings. In some breeds horses with all-over roaning and a blaze (or having even less white on the face) have been tested to carry Sabino1, the only form of sabino that can be tested at this point. Belgians are not known to carry Sabino1, though it’s also true that testing has not been widely done among many of the draft breeds, with the exception of the Gypsy Horse.

What we call sabino is in fact a lot of different types of patterns. Those patterns can be sorted out visually, as I did in the upcoming book, but it may be that those visual categories include different genetic colors that sometimes mimic each other.

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The limits of visual identification

743px-Schwarzwaelder-hengst

As genetic testing has become more common, and there are tests for more and more colors, one thing that has become apparent is that visual identification is not foolproof. Sometimes colors and patterns look alike, but prove to be two different things. The characteristic coloring of the Black Forest Horse is a good example of this.

Many have identified the horse in the picture above as a black silver. He is in fact a chestnut. In his case, there are clues that he is not actually a silver. Although the lighting in the picture makes it less obvious, his coat has red undertones that are more typical of a chestnut than a black-based silver. Even more noticeable is the red coloring at the top of his tail. Visually, it would be possible to guess he is not a silver. There are, however, a lot of Black Forest Horses that don’t give so many clues.

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The Black Forest Horse was the subject of a study that determined the stallions currently standing were all genetically chestnut. (Stallions from each of the sire lines can be found here.) Even more interesting, some portion of them carried the rare alternate form of chestnut (ea) previous only known in the Asturcón and other primitive Iberian breeds. There was no correlation between the alternative chestnut allele and the distinctive flaxen-maned liver of the Black Forest Horses, though. Whatever causes the horses to be so dark, and to have such bright, contrasting manes is not currently known. It would be interesting to find out, because the horses have the same look as black silver without some of the downsides. Reports are that the Black Forest Horses keep their contrasting manes and tails over their lifetimes, a claim that is supported by the appearance of many of the older breeding stallions. Silvers, on the other hand, tend to have manes and tails that darken somewhat with age. Perhaps even more appealing, the coloring on the Black Forests is not linked with Multiple Congenital Ocular Anomalies (MCOA, formly known as ASD), an congenital eye defect thought to be caused by a gene residing close to the silver gene.

As more tests are developed, we are likely to find more and more of these look-alike colors and patterns. In the meantime, it is still fun to speculate about what might be found!

(All pictures in this post come from Wikimedia Commons.)

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