Dear Dr. Sponenberg,
Your name always comes to mind when in the pursuit of answers to difficult genetic decisions within our organization, the American Bashkir Curly Reg. While most breeds have been made and manipulated by man, this breed has insisted on replicating its many good qualities all by itself through its unique dominant Curly gene syndrome. Perhaps belatedly, and with concern we are seeking better understanding of genetic patterns and bloodlines before rewriting breeding regulations. We have closed our Registry as of 1991, and only allow foals of Registered Full Curly parents to be registered as Full Curlies, except for a Grandfather clause which allows one parent to be a mare that is registered as a Half Curly Straight with a registration number under 375.
A Half Bashkir Curly Division was also formed at this time as a vehicle was needed to continue to identify and respect all other Curly Haired horses, as well as an increased awareness for a larger breeding pool. We as breeders and Board Members are beginning to realize that many genetic questions continue to arise and that responsibility in breeding can only occur with education. I have tried to devise a questionnaire for this purpose. Would you please give us your answers and educated opinions on the following:
1) Is there a dominant Curly Gene?
a) Can a dominant gene ever be recessive?
I have seen in poorly expressed, which is not the same as recessive. Say surprises can happen.
2) How can two straight horses have a Curly?
The recessive curl is a separate gene. That is, there are two difference mechanics for curl—one dominant, one recessive.
a) What breeds carry this recessive gene, and is it rare?
Foxtrotter, Percheron, maybe others.
b) If you breed two recessive Curlies together, will you get a Curly offspring?
c) Would this offspring possess the characteristics of the dominant Curly individual?
Not genetically. It would look nearly the same. The confusion seems to be that there are two systems controlling curliness:
Curly dominant with straight recessive
Straight dominant with straight recessive
These two systems have nothing to do with one another, except that the mutant type (curly) is similar in appearance. So here are the gene types:
System 1 System 2
Homozygous dominant Curly Straight
Heterozygous Curly Straight
Homozygous recessive Straight Curly
d) Would it be beneficial for breeding purposes to identify this type of horse in Registry records and to have a special section for this individual, or should we even register it?
Recessive curlies are already in the Registry.
e) Should you breed a Dominant Curly to a recessive Curly?
No real advantage.
f) What are the possible results of such a breeding?
About one-half Curly, one-half straight.
3) What does heterozygous mean?
It means that the gene pair is made of unlike genes.
4) What does homozygous dominant mean?
It means that the gene pair consists of two dominant genes.
a) What is homozygous recessive?
The gene pair is two recessives.
5) Can two Curly horses have a straight foal?
a) Does a straight offspring of two Bashkir Curly parents have any Curly gene and is it more likely to produce a Curly foal when bred to a Curly than any other straight horse?
If the parents were the recessive sort of Curly.
b) Would it be beneficial to register this horse for breeding purposes as it has a strong Curly bloodline?
For clarification of terms, the word Curly or Bashkir is being used to denote those individuals believed to possess the dominant gene, and recessive gene animals will always be identified by the abbreviation Rec. placed before Curly.
6) The decision to close the Registry was based on the opinion that this would strengthen the breed and, in so doing, minimize the chances of having Straight-haired offspring following careful selection of just the right stallion and eleven months of anticipation. Should our focus be directed to bloodlines or to breeding Curly to Curly to accomplish this?
Probably Curly to Curly, with careful attention to keep enough bloodlines.
7) Our Registry was formed in 1971 when a small nucleus of people in Nevada owning or working with this rare animal started comparing notes and came to the realization that these twenty-odd horses all shared common and unusual characteristics. The quiet people-loving temperament was combined with dense bone and forward way of going a well as erect head set. Mane and tail shedding was unique with this breed. It would be interesting to know if this phenomenon also exists in the recessive curly horse.
I don’t know.
History has revealed that these Curly horses were among the wild herds of the West prior to 1700, and as a horse is wont to do, bred only by natural selection. Beginning around 1960, the Curly horses found on the Damele Ranch in Nevada were crossed with different breeds, including the Appaloosa, a Saddlebred, an Arabian stud as well as a Morgan more recently. The offspring, when Curly, continue to bear their own distinct characteristics. How can the one gene carry all these characteristics?
This is unlikely but could be.
8) Is a horse with both Curly genes (homozygous dominant) any different than a horse carrying only one gene (heterozygous), other than it will always produce Curly foals?
This is uncertain.
9) Some concern about diluting our Bashkir blood by continuing to breed to other breeds has been expressed. Is the genetic makeup as important? More important? Less important than in using bloodlines to determine the integrity of the breed?
If the idea is to produce 100% Curlies, then genetic makeup is important.
a) Is it possible to dilute a gene?
b) Sport Horse breeding is on the increase in the US, and some of our Bashkir horses have excelled. Their successes have resulted in requests for crossbreeding. As marketability is necessary for breed preservation, would quality Curly animals that result from this crossing be useful for better selection of breeding stock?
Depends on which way you want to go. The Curly trait in your horses almost certainly came from Spanish horses. The Bashkir is no longer purely Spanish, though, so the breeder needs to pick which direction to go in. Sport Horse is one such direction, although I suppose it will take outside blood.
10) Is there any way of testing to know if your curly has both Curly genes and is a homozygous dominant?
No, other than through breeding.
11) Would you please demonstrate by a diagram what breeders can expect when breeding:
a) If you breed a homozygous dominant to a straight, what percentage of Curly foals would you get?
b) Will all of those foals be heterozygous?
c) If you breed a heterozygous horse to a homozygous horse, what percentage of their foals will be homozygous?
What percentage will be heterozygous?
Will any be straight?
Should not be.
d) How would a breeder know if their stallion or mare is homozygous?
Test breeding to Straights for about 7 foals. If all are Curly, then the horse is probably homozygous.
e) Is there a possibility of scientifically testing for this in the near future?
Potentially by DNA testing.
f) If you breed two Curly heterozygous horses, what percentage would be Straight?
What percentage would be Curly heterozygous?
What percentage would be Curly homozygous?
12) If you breed two Curlies and get a straight foal, does this indicate that both parents are heterozygous?
Yes (or one of the recessive type, although this is nearly limited to the Foxtrotters).
13) In reviewing these questions and answers, would your conclusion be that this breed is determined by genetic factors versus bloodlines?
14) We as breeders are aware that most horse breeds are determined by bloodlines. Are there any other horse breeds determined by genetic factors?
Palomino, Paint, Appaloosa, White
Are there any other breeds of animals that use genetic factors?
15) Reviewing the answer to question 11d, and with the understanding how important it is to develop quality homozygous breeding stock, how many generations would you advise of breeding heterozygous Curlies to heterozygous Curlies (those that are registered in our Half Bashkir Curly Registry) before their offspring are advanced to the Full Curly Registry if each generation is bed only to Curlies?
Two generations, although some will still slip through.
a) As we are unable as yet to determine by blood test if the Curly gene is homozygous, what percentage of chance should we anticipate for the possibility of a homozygous offspring following two generations of breeding from two heterozygous individuals when each generation is bred Curly to Curly?
What percentage chance in three generations?
b) What if a horse of the first generation was bred to a homozygous dominant animal, what percentage of chance would their offspring have of being homozygous?
c) What would be the chance for a homozygous individual if one of the Curly parents was also homozygous in the second generation?
16) If a blood test is developed that reflects the homozygous gene, would you suggest this be utilized before an individual could be advanced to the Full Curly Registry?
Yes, although I would suggest that any homozygous horse be fully registered. That way people could use outcrosses to bring in what is necessary.
a) If the above test becomes available, would it be advisable to require this identification for all breeding stock?
We appreciate all the time, changes, rewording, and most of all, your willingness to share your scientific expertise and good judgment in helping us be “the best we can be” as breeders. Please feel free to add your thoughts and ideas as well as suggestions. If more information is needed, please feel free to call.
Looking forward to seeing you at the next Spanish Mustang Convention in ’76. My husband and I have been invited as it is being held at our good friend, Carl Peters, by the Hoosier National Forest. I hope you bring your Curly Mustang!
I will be unable to come to the SMC meeting.
PS. Do you think that as of now advancing a horse from the Half Bashkir to the Full Curly could be done by parentage of chance of foals being homozygous—i.e., a 75% chance? Blood typing of homozygous dominant would be so much easier.
Blood typing is the best.
By: Sandra B. Hendrickson
Submitted to: Dr. Sponenberg Nov. 13, 1995
Editor’s Note: Dr. Sponenberg, DVM, PhD, is from the Virginia-Maryland Regional College of Veterinary Medicine in Blacksburg, VA