Grey and roaning - help me make sure I understand

[weixiao]

My basic understanding of grey is as follows:

Het. gray parent = Gg and the foal has a 50% chance of inheriting G

Two het parents = 75% chance of inheriting G

Hom. Parent = GG and 100% chance

If foal isn’t grey, foal doesn’t have G and grey will not be expressed.

My family has a chestnut mare who is home bred. She is by a grey stallion who had a big star and snip and a hind sock (who only had two foals - he was my YR horse and the people who bought him from me haven’t bred him. Other foal was grey with a hind sock out of a solid dark bay Cleveland Bay X mare who was a major white suppressor, it seemed.) and out of a sport bred TB mare who was bay with a small white star. I don’t know what colors her parents were.

Chestnut (which I understand often allows for more white expression) has a blaze that extends to her lower lip, hind stockings, a front sock, and a front sock that is only to mid fetlock in front of the leg and to the back of the knee behind. She also has extensive roaning throughout her coat (but no white in the tail). She is not grey/greying.

I think it is funny that we unexpectedly got so much white. However, when I have mentioned it, I’ve had several experienced horse people tell me that the roaning is because the stallion was grey. I thought that since she is not grey she doesn’t carry G and the grey gene and the gene that would cause the roaning are two different things. Am I mistaken?

[MuskokaLakesConnemaras]

Likely rabicano or Sabino at work if the mare in question isn’t grey.

[vineyridge]

Are you sure your numbers are right?
Seems to me that with one Gg parent and one gg parent, the foal only has a 25% chance of being gray. With two Gg parents, the foal has a 50% chance of graying. With one GG parent, the foal has to be gray, but may be either homozygous or heterozygous depending on the other parent’s contribution.

[epowers]

[QUOTE=vineyridge;8153623]
Are you sure your numbers are right?
Seems to me that with one Gg parent and one gg parent, the foal only has a 25% chance of being gray. With two Gg parents, the foal has a 50% chance of graying. With one GG parent, the foal has to be gray, but may be either homozygous or heterozygous depending on the other parent’s contribution.[/QUOTE]

You are confused. If one parent is Gg, bred to gg, they have a 50% chance of passing G and 50% chance of passing g, giving 50% chance of a grey foal with each breeding. With two Gg greys, it is a 50% chance of being Gg, 25% chance of being GG and 25% chance of gg; so 75% chance of grey and 25% chance of non-grey. This can be checked very simply with your basic punnet squares.

[JB]

@vineyridge;8153623

Are you sure your numbers are right?
Seems to me that with one Gg parent and one gg parent, the foal only has a 25% chance of being gray.

50/50 - Gray is a complete dominant, meaning only 1 copy has to be there to do its thing. The Gg parent has a 50/50 chance of passing G, for a gray foal

With two Gg parents, the foal has a 50% chance of graying.

Each parent has a 50/50 shot of passing G. That means there is a 50% chance of Gg gray (1 parent passes G), and a 25% change of GG gray (both parents pass G), and a 25% chance neither parent passes G. The sum means that Gg x Gg gives you a 75% chance of gray.

With one GG parent, the foal has to be gray, but may be either homozygous or heterozygous depending on the other parent’s contribution.

Correct.

[JB]

[QUOTE=weixiao;8153347]
I’ve had several experienced horse people tell me that the roaning is because the stallion was grey.[/QUOTE]
Being an experienced horse person doesn’t remotely mean they are experienced with color genetics Heck, many vets don’t know color genetics.

thought that since she is not grey she doesn’t carry G and the grey gene and the gene that would cause the roaning are two different things. Am I mistaken?

You are correct

Roaning or “roaning” or “ticking” are different from gray. Roaning is really relegated to true Roan, or the Appy varnish roaning (from Varnishing, not Roan). Sabino and Rabicano can cause “roaning” or, more correctly, white ticking in the coat - a little or a lot. Heavy rabicanos can have measurably more white hairs in their coat in the barrel section than the dark hairs. Rabicano likes to stop at the shoulders though, whereas Sabino ticking can be all over.

If there are no white hairs at the top of her tail, then you’re likely looking at Sabino or some other white mutation ticking, not Rabicano, and definitely not gray in this case.

[weixiao]

Thanks for the replies, everyone!

JB- I hear you. I just didn’t want to disagree with them until I was sure that I knew what I was talking about (at least to a point!)

[whitney159]

I’ve been tempted to post something about my yearling on this topic. She is a chestnut with two white stockings in back, a very wide blaze with white extending to her bottom lip (which the sire has as well). Mom has very little white at all (star) but a little bit of scattered white hair mixed in on hips and shoulders, but VERY little.
When the baby started to shed her coat this year, more and more white hairs started showing up, a lot mixed in on her croup, some on the shoulders, very little on her neck and head (very close to none)…but freakishly, she is more white than chestnut in her armpits and groin area and running down the inside of her hind legs. She’s been slow to shed all the old coat (had a coat like a beaver pelt this winter and has mane and tail like a Thelwell pony) and I was going to post pictures in the next week now that we are finally about down to the slick coat and you can really see just how much of that roaning she has. With no white at all a the top of her tail, I’m thinking Sabino. I’ve never seen a coat quite like hers.

[JB]

That “armpit white” is not uncommon with some White patterns (aka Dominant White). It can also come from Rabicano, as does the groin/flank ticking very often. Since you say there’s really no white at the top of her tail, it’s more likely to be some version of White or Sabino.

[whitney159]

Thanks JB! From now on, I’ll pay more attention to other chestnuts and maybe I’ll notice more with the same pattern this filly has. I was leaning toward Sabino more because the other solid white pattern she has seems to be textbook Sabino. I think I might change her name to “Frosted Flake” now.

[JB]

Keep in mind that W20 is a White pattern that can produce a phenytope that is exactly like what we used to always consider Sabino