You don't need stacking. There is no stacking actually happenning at all. I'll try to explain it very briefly. There's enormous amount of people who already transfer a gene of certain disease, but that disease does not manifest upon those people in any form, they are just and only transfering gene of it. Let's tag such gene of transfering person as "Aa". A is the "fine" part of gene and the other one, a, is the "bad" part of gene. If this transferer procreates with somebody completely healthy, or in other words lacking any defect part of gene, you can't spawn anything genetically diseased. Let's tag such person as AA since both his halves of gene do not code any disease currently in question. Which genes will be present in next generation is based on complete lottery. By crossing AA and Aa, each parent gives one half of his gene randomly to each descendant independently. Therefore, there are 4 possible outcomes - AA, Aa, AA and Aa. This means there's 50% chance that descendant will be completely healthy or 50% chance that he/she will trasfer defect gene. Now, if you cross two transferers, which is extremely likely during incest compared to, well, non-incest, crossing will look like this - Aa x Aa -> AA, Aa, Aa, aa. 1:4 chance of being healthy, 1:2 chance of being transferer, 1:4 chance of horrible mutation manifesting. These chances are present in all generations, every time same chances, no stacking. This is the most common case of genetic diseases manifesting themselves, but of course a sick person can spread his gene as well, so by following these rules, you could find out that if somebody with two defect parts of gene procreated with healthy person without any defect part of gene, the next population will not be sick, but there will be 100% chance that all of descendants will be transferers of defect gene. That's more or less all to it.
I did learn about that in high school. :P The chance is there, but realistically, I assume you need some stacking up over time/baby count.
Note that there is a little more to genetics terminology, which I admit to barely remember anymore. Also, this was the case of autosomal recessive disease, which are the most common to appear this way, but there are also autosomal dominant where even person with Aa gene will be "genetically sick" and not a transferer anymore, and there is also gonosomal inheritance which is also slightly different.
Last edited by j4rio on 02-07-14 at 23:30