Hi folks,
I've been out of town for a week and am just getting caught up on my email. I wanted to chime in on the discussion of Barbado and Barbados Blackbelly genetics. I apologize for this email being so long; it addresses several different questions and hopefully will help everyone understand things a little better.
I regularly discuss this issue with people who understand genetics far better than I do (I have very little experience with genetics) and the bottom line regarding the genetics of horns in these two breeds is that even the experts really don't know how to explain it. The Barbado is a cross between several sheep and therefore there may be more than one gene that controls polledness/horns on the chromosome. The Barbados Blackbelly, even in Barbados, often has scurs or small horn buds. Since Barbados seems unwilling to share its breed standards with anyone in the U.S., it is difficult for us to get a clear understanding of what they regard as "scurs" and "horn buds" and how their registry handles the issue. Suffice to say that it is not a simple heterozygous/homozygous, black and white issue. And since no one person or institution has coughed up the bucks necessary to conduct a complete DNA genotype of these breeds, all we have is phenotype to go by (how the animal looks).
Devon asked "How could you find out who has the horned gene?" Since we do not have a genotype "road map" to use, the only way to determine which animal is carrying a horned gene is to test that animal by mating him or her to animals of known bloodline. This is called "progeny testing." Breeders need to conduct progeny testing to ensure that they have either good horned stock or good polled stock.
Here's a good example of progeny testing in a nutshell: A ram (of unknown parentage) is bred to a half dozen unrelated ewes, and all the ewes produce offspring that are acceptable under a breed standard. We then assume the ram to be purebred. If half the offspring are unacceptable, we go back and breed those ewes to a different ram. If the same ewes produce unacceptable offspring, the ewes are probably not pure. We'll try to find some better ewes to breed to the first ram. However, if the ewes all produce perfectly acceptable lambs by a different ram, then the first ram's lineage is definitely in question.
As you can imagine, nothing in real life ever works this easily. Sometimes it's necessary to make some judgement calls, and mistakes will be made. Sometimes one realizes that grandma - and all her offspring - really should be called into question.
So, to answer John's question, the reason that James can be certain that it was his most recent ram that contributed the horned genetics is because in all prior matings to a variety of rams, his group of ewes produced polled ram lambs. However, it cannot be entirely the ram's fault. As they say, "it takes two to tango." To understand why, read on.
Geneticists believe that polledness in sheep is dominant and that horns are a recessive trait. Here is an excellent primer on the genetics of horns in our sheep. It is a very simplistic model and works ONLY if none of the ewes in a flock have horns. It was written by Dave Thomas, Professor of Sheep Genetics and Management, University of Wisconsin-Madison and is printed here with his permission:
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Inheritance of horns is not as simple as the genetic textbooks make it seem. It is complicated by the fact that inheritance can be different in males and females and in different breeds. However, generally speaking, horns are recessive and polledness is dominant. If I let P = the dominant gene for polledness and p = the recessive gene for horns, the following situations can result:
PP = animal has 2 polled genes and is polled
Pp = animal has 1 polled and 1 horn gene and is polled because the polled gene is dominant
pp = animal has 2 horn genes and is horned
If you purchase a polled ram, he can be PP or Pp. If he is PP, all his progeny will be polled because all the progeny will have at least one P gene. If he is Pp, he can produce polled or horned offspring depending upon the type of ewes he is mated to.
Barbados ewes are usually polled, but they can still carry the horn gene. They just don't express the horn gene. Mating a polled ram that is a carrier of the horn gene (Pp) to different types of ewes will give you the following types of progeny:
Ram x Ewe = Progeny
Pp x PP = 50% PP, 50% Pp = all lambs polled
Pp x Pp = 25% PP (all polled), 50% Pp (all polled), 25% pp (males horned, females polled)
Pp x pp = 50% Pp (all polled), 50% pp (males horned, females polled)
If the polled ram has a horned parent, then the ram is Pp. If the polled ram has produced lambs and at least one was horned, then the ram is Pp. If the ram has been mated to several ewes in a flock where horned males are found and he has never produced a horned lamb, then you have some confidence that he is PP, however, there is still a chance that he is Pp.
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In James case, because polledness is dominant, the ONLY way his ram could have produced horned lambs is by mating with a ewe that was Pp. So, you can now see how tricky it is to figure out what genetics a flock really contains.
This brings me to the question of what do we do with animals that are heterozygous (Pp)? What do we call them? Are they Barbados Blackbelly or Barbado? The BBSAI breed standards ONLY apply to what an animal looks like--its phenotype. If a ram doesn't have horns, the BBSAI will currently register it as Barbados Blackbelly. The BBSAI is investigating whether or not it wants to impose progeny testing on candidates for Barbados Blackbelly registry to eliminate some of the animals that carry the Pp genes. If a ram has horns, the BBSAI will register it as Barbado (or whatever name the membership votes in September to use to distinguish this breed). However, a horned ram may produce polled ram lambs, depending on what ewe he is bred to. Regardless of the phenotype, breeders MUST be completely honest about the genetics of their flock. Because this is an ongoing learning experience, breeders cannot always be certain of the genetics and must clearly indicate the risk level to a potential buyer.
For reputable breeders of both Barbado and Barbados Blackbelly sheep, determining the genetics in a flock requires a series of trial-and-error tests. But because our ewes are polled and can carry that hidden horned genetic, it is tremendously difficult to establish a truly homozygous flock of 100% polled sheep or 100% horned sheep. The BBSAI is tightening its breeding standards, clarifying the difference between the two breeds, and launching an effort to educate all breeders about the difference. It is vital to both breeds that this difference is clearly understood. For the Barbados Blackbelly, without immediate intervention the breed will soon become extinct in the U.S. Breeders MUST understand that if they have horned ewes or rams, they do NOT own Barbados Blackbelly sheep. For the Barbado, it is critical to eliminate polled genetics to ensure good horn growth and "true" horned bloodlines. We must work to eliminate the confusion between these two breeds' identities and absolutely SEGREGATE the bloodlines.
Carol Elkins Critterhaven Estate Registered Barbados Blackbelly Hair Sheep (no shear, no dock, no fuss) Pueblo, Colorado http://www.critterhaven.biz T-shirts, mugs, caps, and more at the Barbados Blackbelly Online Store http://www.cafepress.com/blackbellysheep
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