# Homozygous Roan Lethal?



## Bridgertrot

So I'm currently taking a genetics class. First day was today and she put up some extra credit already for us to do. Lucky for me it's all about horse genetics YAY! I might need more help when I start into it more. She gives us pictures and we're suppose to say what it's full genetic coding is...

Anyway...I'm reading the powerpoint and according to it: "All roan horses are heterozygous Rnrn as homozygous RnRn is lethal".

Is this true? I've actually never heard of this. Does it work the same way as lethal white? If I thought about it correctly this would mean breeding two roans would have a 25% chance of lethal. Am I totally crazy or is this incorrect?


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## Poco1220

I've never heard of such a thing in roans...
_Posted via Mobile Device_


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## Bridgertrot

Yeah that's why I'm asking. I pretty much did this when I read it:









LOL

I need some of the other color smarties so that if this is indeed incorrect (like I feel that it is) I can go to my professor and be like "Y .U NO TEACH RIGHT?!"


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## NdAppy

Roan used to be thought of as embryonic lethal, but for the type of roan that is testable now that has been proven wrong.

This stud is homozygous for roan - 

MESA T RANCH


And this is from the UC Davis page - Roan



> Roan is a white patterning coat color trait of intermixed white and colored hairs in the body while the head, lower legs, mane and tail remain colored. Roan horses are born with the pattern, though it may not be obvious until the foal coat is shed. The white and colored hairs are evenly mixed in horses that inherit the classic Roan gene, which can differentiate this from several mimic patterns called *roaning*. Roaning patterns tend to be uneven in the distribution of white hairs and the inheritance of roaning has not been defined. The Roan gene is found in a variety of breeds such as Quarter Horse, Paints, Peruvian Paso, Paso Fino, Welsh Pony, Miniature and Belgian, but not in Thoroughbreds or Arabians.
> 
> 
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> *Although it has been suggested that Roan is a homozygous lethal, evidence from the Quarter Horse breed indicates otherwise. Production records have documented the existence of Roan Quarter Horses that produce 100% Roan foals. DNA tests have confirmed homozygosity in the genomic region that contains the Roan gene. *


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## Bridgertrot

Hm...excellent. So would it be worth sending that article to my professor?


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## Faceman

Some professor...just reinforces my lack of respect for academia. Years and years ago, people used to think a "double roan" was lethal, just like a "double white" as they used to call them. It was an old wives tale that I haven't heard come up in 30 or 40 years...as many roans as are crossed with each other you would think that belief would have fizzled out long ago. I suspect your professor got it mixed up with lethal white...

Yes, you should tell her...and give her a homework assignment...:lol:


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## TexasBlaze

From what i understand it is 97% lethal. Sometimes some lucky horse is strong enough to be born Homozygous but its very rare. There is one line though i cannot remember what line it is that throws a lot of Homozygous roans. Though i could be totoally wrong. Thats just what ive read.


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## NdAppy

I would pass that information along to her. 

You wouldn't believe how many times I wanted to drive to where Po was taking her color classes and smack the teacher in the back of the head...


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## NdAppy

TB, if possible, please do share with us that information if you can find where you read it.


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## Poseidon

NdAppy said:


> I would pass that information along to her.
> 
> You wouldn't believe how many times I wanted to drive to where Po was taking her color classes and smack the teacher in the back of the head...


Yes, please send it to her. 

My professor hated me during the color section of my class last year because of my constant corrections (ie, "White." There are quite a few ways to get an entirely white horse, not "greys have dark skin, whites have pink skin." GAH!). I sat texting NdAppy furiously under my desk because everything was so wrong.


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## TexasBlaze

This is what i found. 

*the roan gene has been idenified

*


Roan, along with dominant white, sabino-1 and tobiano have all been mapped to the KIT gene region on Chromosome 3. KIT is a large complex protein involved in many important aspects of metabolism. Some KIT mutations are homozygous lethal, including some of those causing dominant, of which there are more than one, in different horse populations. 

*As with dominant white there may be more than one roan allele, some may be homozygous lethal, others not.* 

roan horses


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## Poseidon

wut. 

That website uses a study from _1979_. I think we've made a bit of scientific progress since then.


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## Bridgertrot

What all should I include in the email I send her? Just mention that the information is not correct and provide her the link to UC Davis or? Makes me wonder what else is incorrect that I'm not noticing. 
_Posted via Mobile Device_


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## PhelanVelvel

First of all, I think it's awesome that you identified that piece of information as incorrect, and you definitely should tell her. In my Conservation Biology class my professor knew less about certain topics than I did, mostly because I get very obsessive over certain facts. She also told me I was wrong when I answered a question and I knew I was right beyond any doubt so I had to e-mail her and set her straight. Just goes to show that a PhD helps, but it does not mean everything.

If I were you, I would just explain that the belief that "homozygous roan is lethal" was founded on a paper from the 1970s--they thought that it was lethal because there were fewer roan offspring from roan to roan matings than were expected based on what they knew about genetics at the time. This lead them to conclude that it was lethal to the embryo, but it has actually been shown that it's less straightforward than that. There are living horses which have been shown to be homozygous roan.

Don't know if anyone has posted this link yet: http://www.hancockhorses.com/article-roanQHNews.pdf

It gives some good information on the findings by UC Davis and discusses the "forthcoming" paper. I can't find the paper anywhere, is it just not published yet?


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## Saddlebag

Don't tell her her info is incorrect. Suggest to her that you are confused by all the information you've been given both by her and what you've read. That way hopefully she won't take offence. She's the one who grades your papers.


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## Chiilaa

PhelanVelvel said:


> If I were you, I would just explain that the belief that "homozygous roan is lethal" was founded on a paper from the 1970s--they thought that it was lethal because there were fewer roan offspring from roan to roan matings than were expected based on what they knew about genetics at the time. This lead them to conclude that it was lethal to the embryo, but it has actually been shown that it's less straightforward than that. There are living horses which have been shown to be homozygous roan.


I want to elaborate on this because it is a very good point. Roan is a very complex simple dominant lol. It is a mutation of the KIT gene, which automatically makes things complicated. There are currently four KNOWN and TESTABLE mutations of the KIT gene - tobiano, dominant white, roan and sabino 1. This means that only two copies of ANY of these genes can be inherited. So we can assume this horse is heterozygous for both tobiano and roan, and does not have sabino.










The KIT gene is also linked to the extension gene, to the point where they inherit in pairs. If, for example, you had a Ee Toto stud, let's assume the E is linked to the To and the e to the to for this discussion. His foals will inherit either E To or e to, never e To or E to.


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## Celeste

If you can find documentation that the teacher is in error, you should bring it forward. But do it in a nice way. This person is highly educated, and if you come across as arrogant, she will no doubt embarrass you severely. Nobody knows everything. The entire genetic plan of all horses has not been firmly established or proven. She will welcome intelligent discussion. She will hate arrogant crap.


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## texasgal

He's RnRn .. tested.

Looks pretty good for a dead guy, huh?


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## Faceman

Bridgertrot said:


> What all should I include in the email I send her? Just mention that the information is not correct and provide her the link to UC Davis or? Makes me wonder what else is incorrect that I'm not noticing.
> _Posted via Mobile Device_


I would include a link to UC Davis - the ackowledged experts on the subject.

Roan



> Although it has been suggested that Roan is a homozygous lethal, evidence from the Quarter Horse breed indicates otherwise. Production records have documented the existence of Roan Quarter Horses that produce 100% Roan foals. DNA tests have confirmed homozygosity in the genomic region that contains the Roan gene.


Thus, there is both observational and genetic evidence that is contrary to her statement...


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## NdAppy

Celeste said:


> If you can find documentation that the teacher is in error, you should bring it forward. But do it in a nice way. This person is highly educated, and if you come across as arrogant, she will no doubt embarrass you severely. Nobody knows everything. The entire genetic plan of all horses has not been firmly established or proven. She will welcome intelligent discussion. She will hate arrogant crap.


So is that how you treat your students? Make their likes a living heck because they present information to you in a way that makes you feel like an idiot? That's a real mature response and anyone who treats there students that way really, really needs to reevaluate why they are teaching to begin with as that is wrong and totally unacceptable behavior from ANYONE that is teaching regardless of the ages of the students.


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## Celeste

Lol. No, I don't treat my students that way. I have, however, had teachers that did..........


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## nuisance

This guy use to belong to a friend of mine. She sold him to a breeder down near Austin. He's tested RnRn, throws 100% roans. Hat Creek Shane Quarter Horse


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## Bridgertrot

Alright, I sent her an email, hopefully I'll get a nice reply back! 

On a side note, I'm having trouble with a Dominant White foal picture.










I still struggle a bit with some. It's obviously Ww but what about E and A (agouti)? I think I see some chestnut on the ears...is it safe to say that this is ee aa? And would you name this a Dominant White Chestnut?


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## NdAppy

I would have to put him down as red based, with agouti unknown. You can't tell agouti status of a horse except in a very, very few cases.


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## CowgirlRocky

*Another Homozygous Roan Stud*

Here is a Tennesse walking horse stud that is a homozygous roan. His raon zygosity test is Rn/Rn.

Homozygous Blue Roan Tennesssee Walking horse, Generator's DR Blue,


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## ThirteenAcres

Bridgertrot said:


> Yeah that's why I'm asking. I pretty much did this when I read it:
> 
> 
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> 
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> 
> LOL
> 
> I need some of the other color smarties so that if this is indeed incorrect (like I feel that it is) I can go to my professor and be like "Y .U NO TEACH RIGHT?!"


I want to note my enjoyment of Snape reference.


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## nikelodeon79

Chiilaa said:


>


I really have nothing to add except I'm DROOLING over this horse!!!!


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## Bridgertrot

NdAppy said:


> I would have to put him down as red based, with agouti unknown. You can't tell agouti status of a horse except in a very, very few cases.


Agouti meaning he may or may not be bay as well right? Because horses with dark points need to be at least AA or Aa right? 

Just trying to do well in this assignment and learn some things! Lol
_Posted via Mobile Device_


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## NdAppy

Agouti is the bay/brown/wild bay modification. A red based horse can carry the modification but will not show it. It only affects black horses. 

There are only two base colors in horses. Black and red. Everything else is a modification of those two colors.


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## NdAppy

Wix.com Equine Color Genetics created by ruffriderdesign based on Close Up | Wix.com


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## Chiilaa

I am another who would mention it to the lecturer. I wouldn't stand up in a lecture and yell out that s/he was wrong. However, I would send them an email with the link to the UC Davis site (UC Davis is a reputable source) and a brief note that you came across this while researching for his/her class and instantly thought of how interested s/he would be in this new information. Should they then continue to spout out of date, incorrect roan inheritance information, I would then call them out in a lecture. 

As far as who marks your assignments, there is always a tribunal system where you can have someone else moderate your assignment to see if the mark given is correct.


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## LuvMyPerlinoQH

I wanted to add the Hancock and Blue Valentine lines are mostly RnRn


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## blue eyed pony

I have a curious observation based on a stallion in my state.

He is "guaranteed" that IF he throws red, it WILL be roan (and hasn't thrown a non-roan chestnut), but he has also thrown black-based roans (to a red mare, most notably), AND he has thrown non-roans. Now, it is possible that these "non-roans" are actually roan (making him homozygous, thus making it impossible for him to throw non-roan fullstop), and just not expressed... I would be curious to see the results of genetic testing.

The man himself is a "blue roan" who appears to me to actually be brown roan based on tan around his muzzle, very nice looking fellow.

I'm just wondering what might cause an Ee At- Rn- stallion to always throw roan when he throws red, and why some of his black-based progeny are very obvious roans while some do not appear roan at all. And why his red roan progeny are all loud.


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## Chiilaa

There are cases where linked genes have split and been inherited separately. It's unusual, but not unheard of.


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## blue eyed pony

But then shall we pause to consider that most of said stallion's black-based foals (that I have seen pics of) were roan, so it's not an isolated case of linked genes splitting if indeed that is the case. He has thrown multiple blue and bay roans, loud ones, and hasn't thrown a non-roan red (according to owners, who are very reputable breeders), but appears to have thrown non-roan blacks and bays.


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## Chiilaa

I would imagine the split would then be in his own DNA so that they are no longer linked in HIM. This would then cause them to be unlinked in his sperm if this makes sense? Without a test, there is no real way to tell what genes a horse like this has. He would have to have at least 100 foals on the ground to start forming an educated guess.


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## blue eyed pony

Hmm, interesting indeed, but why, then, would he consistently NEVER throw a straight chestnut? If his roan and extension aren't linked, wouldn't that mean he does have a chance of throwing a non-roan chestnut?

I don't know how old he is but I'm pretty sure he's "young-ish", not a stallion in his first few seasons because his owners clearly have a very clear idea of what he consistently throws, but he's not exactly old either.


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## Bridgertrot

So just to make sure I'm doing this correctly...she wants us to list every possible genotype. 

This one is a chestnut tobiano: *ww gg ee aa CC dd rnrn TO- lplp* 
This is a buckskin: *ww gg E- A- CCcr dd rnrn toto lplp

*I got an email back from her today. She said it made her day to have a genetics student who is already so enthusiastic. :lol: If I'm deciphering the email correctly...she says homozygosity has been detected in a few individuals but is unsure if this is sufficient enough to infer if the Rn gene itself is homozygous. She is waiting for someone who works with equine genomics to come up with definitive answers. She believes there is a possibility that RnRn is deleterious which leads to a lower survival rate, since you don't seem to find that many.

She mentioned something about how there are "frequent variation in levels of gene expression and phenotypic effects" Haven't learned about that yet so not sure what that's all about.

Glad she was happy about it!


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## NdAppy

Bridgertrot said:


> ...
> This one is a chestnut tobiano: *ww gg ee aa CC dd rnrn TO- lplp*
> This is a buckskin: *ww gg E- A- CCcr dd rnrn toto lplp
> 
> *...


Top has two cream or one? Cream is normally denoted as CRcr or CRCR or CR/n or n/n for cream. Weird to see it as CC. 

Two is cremelo tobiano and one is palomino tobiano.


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## Poseidon

NdAppy said:


> Top has two cream or one? Cream is normally denoted as CRcr or CRCR or CR/n or n/n for cream. Weird to see it as CC.
> 
> Two is cremelo tobiano and one is palomino tobiano.


My professor did the EXACT same thing and docked me points when we had a test with a listed genotype and had to name the color. I had random answers like perlino dun for something that was supposed to grulla. I told her it didn't make sense to have a capital letter meaning the gene is not there when all of the other letters were lower case.. I don't even know.


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## Bridgertrot

See that's where I'm confused too. I would figure two cream would be CRCR, one CRcr and none crcr. But I'm following what it says on the powerpoint I've been provided.

"-Codominant allele Ccr dilutes red pigment to yellow.
-Fully pigmented red horse is CC at this locus.
-Heterozygous CCcr dilutes bay to buckskin and chestnut to palomino but does not dilute black.
-Homozygous CcrCcr dilutes ALL colors except white or gray to pale cream (cremello) often with pink skin and blue eyes."

Wonder if I can get away with just label it CRCR etc.

The first horse I posted is just a plain ol' chestnut tobiano. Though I think she has frame due to her jagged markings (which I'm putting on the assignment LOL) 

It's like they add more letters than they need to for identifying cream x.x

​


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## NdAppy

ok that is just confusing as all heck. Po was that how yours was? I still want to smack that teacher...


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## verona1016

Using a "C" for no cream present and "Ccr" for cream present is really confusing 

What does she use for no champagne present?

Also kind of silly to have to list all the things that aren't present... you'd have a list a mile long if you included everything that can be identified currently (silver, pearl, champagne, splash, sabino, etc, etc.)


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## Poseidon

NdAppy said:


> ok that is just confusing as all heck. Po was that how yours was? I still want to smack that teacher...


Yep. It was so half-assed backwards. I'm at least relieved to see that there are other people that do it.. Crcr would make much more sense for 1 copy, crcr for no cream, and CrCr for double cream. Le sigh.


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## NdAppy

But, but po! That would make SENSE! and we can't have that!


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