Those Mysterious Modifiers: Umbros (Umbrous)

A while ago I had a French Angora litter with two chestnut and three chocolate agouti bunnies.   Mom is a black based fawn and dad a chocolate agouti. 

The three chocolate agouti bunnies grew up to be chocolate agouti adults, the color on all three look about the same.  

The two chestnut grew up to be very different looking.   If you go back to the top photo, they looked about the same as days-old bunnies but now very different.   

This photo shows the back of the two chestnut that have very different colors.  

I had a discussion with our color genetics guru Candy Haenszel, here is her assessment:

"It looks like your chestnuts go from the extreme one way to the extreme the other way.  The more yellowish one is the sandy type of chestnut, and is caused by wide-band genes.  I don't know if a rabbit has to have two wide-band genes to be expressed, but I think so.  Wide-band genes widen the yellowish tan band on an agouti.  I can see in the photos, that the dark gray band is a lot wider on the wild gray, and the tan band is a lot wider on the "sandy" chestnut.  Wide-band genes are a wonderful thing to have in fawn and cream, because non extension genes usually leave just a faint grayish color band, and wide-band genes make the yellowish color take over more of the wool, getting rid of the grayish, and making a much better/cleaner fawn/cream.  What I have always heard, is that wild gray is caused by umbros darkening modifiers.  They darken the grayish band of a chestnut into a very dark bluish gray color.  I don't know if they also widen the grayish band, but they must, or that dark bluish gray color wouldn't almost take over the whole color of the wool.  Maybe there is another modifier that is closely linked to umbros that widens the grayish band, or narrows the yellowish band.  I don't know.  I just know that umbros modifiers darken the grayish band into that very dark color.  I wish I knew more about how that grayish band is widened."  

According to a summary in the "Journal of Genetics" on

https://link.springer.com/article/10.1007/BF02982492

The Umbros darkening gene only expresses on agouti patterned mice, no visual effect on non-agouti mice.   We are not sure whether such can be applied to rabbits.  If applied to rabbits, then Umbros would not be a viable modifier that darkening my tort bunnies.  Then what is the modifier that accounts for the orangey wool and grayish wool on the tort bunnies? I don't know.   There are many modifiers out there that we don't understand.   We'll conclude by saying "those darn mysterious modifiers"!