A discussion of equine roan color genetics
by Donna Vickery
Can a horse be homozygous roan?
|Hintz, H.F. and
VanVleck, L.D., published 1979. Lethal dominant roan in horses. Journal of
Previously thought not. Basically, the study referenced above says no... the results of which have long been accepted... this study showed that roan to roan breedings produced 67% roans, and 33% non-roans. This is consistent with the homozygous lethal roan gene theory. This particular set of percentages is almost always associated with a lethal gene. If it were a non-lethal condition, there should be 75% roan (1/3 of those, or total of 25% homozygous roans) and 25% non-roan babies.
UPDATE: There is now a new study done by the late Dr. Ann T. Bowling. The geneticists at UC Davis are preparing it for publication, but don't say exactly when it wll be out. There is an article in Quarter Horse News - the December 15, 2004 issue - on roan color genetics, (see it in its entirety here on HancockHorses.com linked on the articles page) a kind of lead up to the release of this study that does acknowledge the existence of homozygous roans in the quarter horse breed. The UC Davis study refutes the earlier findings of the H.F.Hintz and L.D.Van Vleck study done in 1977.
(The article below was written in Oct. 2003, before I knew about Dr. Bowling's study.)
Are some Hancock stallions homozygous for roan?
Per an email discussion on this subject with a lady who has studied the statistics in our modern day roan Hancocks for some time... she proposes that the roan gene is (you pick a number:) 95%? lethal... IOW, that occasionally a homozygous roan embryo is viable, does survive, and lives to produce all roan foals. We do have to see an extraordinarily large number of all roan offspring however, to suspect a horse could be homozygous roan. To my knowledge, (to date) this rule breaking phenomenon has only been documented in a few Hancock bloodline Horses (?) ... time will tell.
However, it is true, that some Hancock stallions, so far, have been breaking the lethal roan gene rule. Wrangler Joe Hancock (deceased) sired over 150 foals, all were roans, 0 non-roans. Blue Apache Hancock has sired over 250 foals, all have been roans, 0 non-roans. These 2 stallions have been bred to hundreds of mares, including grade mares - and yes, many were roan mares - but also many were solid color mares, and as yet, not one non-roan baby has been sired by either of them. Also it is interesting to note that there has been an accompanying significant percentage of dry mares each year too, and late pregnancies.
There are apparently other roan Hancock bloodline stallions out there, who have done the same; such as Hancocks Blue Boy (deceased), and others. To avoid challenging scientific findings, we should call horses like these 100% roan producers, either until they produce a solid non-roan foal, or until research identifies and explains the reality of occasional homozygous roan horses.
See: HancockHorses.com 100% Roan Producing stallion list.
How can you tell a grey horse from a true roan horse?
In horses, both true roan genes and grey genes are dominant genes. That is, a horse cannot hide these genes, if it has one [or both] of these genes, it will express and become evident. If it is roan, it has to have a roan parent, and if it is grey, it has to have a grey parent.
Frequently, AQHA registered Quarter Horse are listed as being roan, when they are not roan, but grey. That is because inexperienced breeders can mistake young grey horses for roans, report them as roans to AQHA, and it is recorded that way on their permanent papers.
Responsible roan horse breeders often consider any foal that appears roan born from both a roan and a grey parent, to be grey until proven otherwise (after 3-4 years of age). 50% of the time, they will turn grey. And in some cases - especially horses which are both roan + grey - this cannot be determined conclusively before they are 3 years old.
If you are interested in buying a roan horse, and do not want a grey, make sure neither of its parents are grey. (Remember, the AQHA registration certificate offers no conclusive proof of this, as it is only as reliable as the information reported.)
So, how can you tell a young grey horse from a young roan horse, especially if you do not know the color of its parents?
A grey horse will show greying (white hairs) on its face, in equal or higher density than on its body coat, usually fairly early on. It is progressive, and a grey horse gets whiter each year. Any color horse can inherit the grey gene, but every grey horse will have at least one grey parent.
A true roan horse will have a solid colored head and legs, the dark color legs usually make a stalagmite point up its legs above the knees & hocks, and its roaning (white hairs) will be dispersed over its body. The degree of roaning (amount of white hairs) will vary from horse to horse but will remain consistent over its lifetime to that horse. The roan over its body will vary seasonally, (lightest in spring, medium in summer, darkest in winter). A roan foal is often born looking solid colored. If it is a light color horse, (carrying creme or dun gene) it may not display its roaning until older. The roan foal will show a lot of coat color development with each shedding, but look for slight roaning in its flanks when born, or creamy underpinning, or silver legs. After it sheds once or twice, you should know whether it is roan or not. It may be lighter as a yearling than as a 2 year old, but past that, roan is not a progressive color. When it reaches its permanent adult roan color, it will remain that way throughout its life. A roan horse will always have at least one roan parent.
Now for the confusing disclaimers:
To make things more complicated, a horse can be born a true roan, with the true roan gene AND the grey gene. If so, it will often appear typically true roan for the first 2 or even up to 3 years of age. That is, its head will be darker than its body... but it will begin to show white hairs on its face by age 3 at the latest, and it will continue to grey progressively, and eventually become white like any grey horse will.
More confusion: A very old horse, (even a roan) will begin to grey in its face, but that is honest old age greying.
AND, there are other "roaning" genes, such as the sabino and the rabicano gene which produce other types of roaning patterns and follow other (widely varying) rules... thus we refer to the more typical and consistent roan gene as a true roan.
AND, there is apparently a frosty roan gene, which distributes a lot of roan hair over the horse dorsally - in the mane, tail, and over the top of the horse, as though it has just been snowed on. This pattern is usually still considered to be a true roan gene, but more research needs to be done to confirm this. It will usually still have a solid face and usually solid legs, although some have frosted legs as well.
AND, once in a while, there will be a true roan horse, that for some unknown reason, does show some light hair in its face. It may be the mealy or muley (pangere) gene, or something similar.
AND the Lp gene (leopard pattern complex) that exists in spotted breeds (Appaloosas, Norikers, POA's, Knabstruppers, etc) has a roan pattern variation, but it is referred to as varnish roan, to differentiate it from the true roan gene of non Lp gene horses.
AND mules. Asses or burros have a distinct roan gene that is different from the roan gene in horses. Therefore roan mules may be genetically different and may follow different rules than those discussed above for horses.
In conclusion, if the roan color of a young horse matters to you, you will need to know its parents, or know the breeder, to know for sure what it is.
NOTE: We have gathered a reference list of stallions that are producing (or have produced in its lifetime) 100% roan foals. If you have or know of one, please email its information to us with photo, stats, owner information, etc. Thank you.
See: HancockHorses.com 100% Roan Producing stallion list.
HancockHorses.com webkeeper, Donna Vickery
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