Postby xfactor fan » Wed Apr 08, 2009 1:58 am
The roan embryonic lethal has been quite a question.
To recap.
Breeders of roans (classic roan) have long felt that, and can prove via their breeding records that there is no such thing as a homozygous roan. The theory is that rr is a solid horse, Rr is a roan, and RR is lethal. This is a very early lethal, and mostly shows up in the numbers of roan horses produced.
The expected numbers in crossing two roans Rr x Rr should be 25% rr, 50% Rr, 25% RR, or 1-2-1. The actual numbers work out to closer to 1/3 rr, and 2/3 Rr, with no horses found that were RR.
However Dr. Anne Bowling in her roan study found several stallions that were RR, or pure breeding roans. She died before her study could be published, and the results were released after her death by her husband.
This of course set the breeders up against the science, with the predictable howling from both sides.
Here's the speculation part.
I believe that both sides are correct, and there are two versions classic roan in the gene pool. R1, and R2. Where R2 has a second mutation that moves the lethal gene, so that either R2-R2 is not lethal, or the lethal portion of the chromosomes don't line up exactly allowing the embryo to survive.
R1-R1 is lethal. R1-R2 is not.
Proof of all or any of this is going to have to wait till the genetic testing locates Roan.