Feathering Genes- What do we know?

[RoyalCrownAcres]

I breed vanners (horses that are valued higher when they have heavy feather in real life), and I've recently been going through my stock and realized a few things:

• Some Horses develop "thick" and "full" feathers significantly faster than others (at 3 years old vs. 5 for example. I'm going to start keeping track of this)

• Horses tend to take after the sire in term of when they develop feathers

• mares seem to develop feathers slower/grow them for longer than stallions

• (the previous 2 points could be some sort of observation bias because I have more strict standards with my stallions but still)

• Feathering seems to be recessive and have multiple genes at play?
  I have 2 mare/stallion pairs that I stopped breeding because they continuously produce foals with little to no feather (most all the foals were rehomed,
  so I don't have examples unfortunately). I cross the mare or stallion to a different horse and the foal has nice feather. One example of the minimal
  feather is this guy
  RCA Giant Minus Feather
  
  Neither parent has ever thrown a foal without pretty decent feather (when crossed with another vanner) until this guy. The sire, Spirit, did have a son
  that I rehomed (along with all his foals) because no matter who I crossed him with, his foals always had feather similar to the guy linked above.
  
  I'm interested to see if anyone else has some feedback or anything to add here. Part of me wants to dive into and figure it out, but I have no idea where I would even start lol.
  
  I did find this from Black Oak
  

  Generally, there are four different expressions of feathers. They are considered recessive, I think, however, there is something else going on since there are four expressions. So more like a recessive incomplete, meaning that they'll show something, but they will need two to show feathering and not minimum expression. Plus it also means that there may be two different types of feathering, light feathering and full feathering, which is why we can get medium or half feathering.
  1) Most Minimum: They have something there, but it it resembles overgrown fetlock hair. This means they do carry the feathering gene and if paired with another, they could produce more feathers. See the below two images of minimum versus none:
  Image
  Image
  
  2) Light Feathering: Expressed, but more like an afterthought. Full fetlock coverage.
  3) Medium Feathering: Halfway up the legs.
  4) Full Feathering: All the way to the knees.
  
  If you want to breed them out, avoid breeding number 1's together. You'll have a 25% chance of producing a feather carrier from a single carrying parent. 75% chance if you breed too of these together to get another carrier (that's 50/50 for single gene carrier and 25 for feathering to occur).

  I'm mostly curious as to if anything new has been figured out

[sunandshadow]

I'm interested in this question too, since I also breed feathered horses (mostly thoroughbreds and Belgians).
Here's an example filly who has noticable feather (stage 2) already by 1 year 5 months
https://www.horseworldonline.net/horse/profile/4396673

Then I've got this colt, he has the tiniest start of feather at 1 year 4.5 months
https://www.horseworldonline.net/horse/profile/4396788
And that colt's half brother looks the same at 1 year 5.5 months
https://www.horseworldonline.net/horse/profile/4396715

This mare, her gallery photos show that she has noticable feather development at 1.7 years old that stayed half-developed until she was about 3, but looks fully developed at 3 years 10.5 months.
https://www.horseworldonline.net/horse/profile/4386434

And then we have late bloomer boy here. He had tiny feather at both 1.5 years and 2.5 years, then his feather finally developed to stage 2 at 3 years of age, and finally made it to stage 3 somewhere between there and 5 years. I assume it's not going to develop to full stage 4.
https://www.horseworldonline.net/horse/profile/4274577

[Cypress Creek Test]

I've got a couple of observations that may come in handy- commenting so I know to come back to this topic.

Feathers themselves are recessive. I believe you CAN visually tell if a horse is a carrier. I believe the age of feathering occurring is genetic, but I haven't really paid attention to stallion vs mare feather expression.

Tarpans, Belgians, and Shetlands are the three AC breeds that have feathers- forest horses might? I don't remember.

You could absolutely breed for early feathering development.