Shorthornboy: I am so glad you typed it wrong!! I about fell of my chair!! ;D
DL can best explain it, but here is my short simple version: TH/PHA are genetic defects that affect the survivability of both cow and calf. All TH/PHA affected calves are dead or die shorty after birth. Due to the fact that the calves are deformed (PHA worse than TH) calves usually are pulled or c-sectioned. Which can impact future fertility of the cow/heifer.
Even if your cows are clean, using a carrier bull, you will produce 50% carrier calves, which in turn will create more carriers or affected calves. Thus multiplying the defect in the general population.
Just heard that a shorthorn heifer that was in the running at the junior nationals last year died. She was ill at 5-6 months gestation and when they took her into the university, they c-sectioned a 150 pound PHA calf out of her....she died shortly after due to a toxicity. Wow, does that tell you enough to not use a carrier bull???
Note: This heifer was mated BEFORE a test was around.....and the "hush hush" theory was in effect to hide carrier bulls.
SHB - I will attach 2 articles on TH and PHA to the next post - they are PDFs and they should tell you everything you need to know - plus there are some really gross pictures. Like Show Heifer (SH to her friends!) said these are lethal genetic defects - the calf dies. They are both autosomal recessive (you need to get the bad gene from both parents). With PHA there is also the potential to lose the cow - this is a bad deal.
Here is a couple of pictures of a PHA calf - this is a face that no one can love.
Here are the articles - looks like you already had the one - if you breed a carrier to a carrier or a carrier to a clean you have a 50% risk with each mating of having a carrier offppring - if you are just starting your herd it would be good to start clean...why ask for trouble, extra testing, potential management issues ....JMO
Definately read those articles!! But to clarify: Yes the gene is recessive, BUT if the article stated that the calf would not be a CARRIER if either parent was negative, IT IS WRONG. If either parent is free then the calf can not be deformed and dead (it got both lethal genes from each parent), but if one parent is clean and one is a carrier, the calf has a 50% chance of being a carrier (will look normal, but will carry the defect). I think one of the articles explains this if it is confusing to you!! (And trust me, it can be confusing!!!)
But bottem line: If you use a carrier, you will get carriers at best, dead calves at worst.