Re: Something something pun about Horizon
Oh, certainly some American geneticists do know about it, because they were involved in getting the conviction overturned. However, I fear you've made the same mistake as "experts" are often very prone to doing:
"a well known genetic situation that is called chimerism, rare, but well studied"
Unless a comprehensive, statistically sound systematic study quantifying the occurrence rate of this condition can be cited, all that can be said is that it is a condition that is rarely encountered, and be very clear that as to the reason why.
In this particular case, there was no such data available at the time. To the best of my knowledge there still isn't. Even the references cited by the Wikipedia article on the topic admit that tetragametic chimerism is of unknown occurrence, and could be quite high. What's interesting is that the reference in Wikipedia isn't even a published, peer reviewed paper; it's just a short blog. That probably means that science, officially, is mute on the topic. Yet almost everyone (including yourself it seems) would be absolutely certain that it is a rare condition.
Honestly, you'd think with this unquantified uncertainty lurking in the background of genetics, you'd think that courts should be pretty skeptical of genetic differences being presented on behalf of prosecutions (such as child care / custody cases), or at least have a standard for considering the possibility of a chimeric. But I severely doubt it. The view that "DNA matches are generally correct" is so deeply entrenched in the popular psyche, it takes a lot of imagination to to even think to ask the question, "are we really sure?". If I correctly recall the outcome of the case in the US, the perceived "rarity" of the condition was used as a justification to not alter court procedures for similar cases...
Peer Review? Nope
A major problem is that the way such scientific data is handled in the criminal investigation / prosecution system is that it is strictly procedural. There is zero room for the forensic scientist to deviate, inquire, question the standards. If they've been told, "do this and if it's not a match, you say so", that what they have to do; they've been told, "the procedure is safe, follow it". You don't get peer review of evidence, there's no space for academic curiosity and to say, "hang on a mo, I read a paper on this".
Currently the court system relies on there being sufficient numbers of eventual successful appeals. Though as we've seen with cases like those involving Horizon and also Roy Meadows, the damage done can be devastating and fatal. You'd think that the high possibility of such an outcome would focus the minds of the judiciary into getting a better grip on how they run their courts w.r.t. technical / scientific data.