Creatures Caves

Working on some nice genetics for my breed some questions came to my mind..

I read in on thread here (it was about colortrue norns) that adding genes somewhere into the genome is a bad idea because in the breeding process the interchange is based on the numbers. If I put a new gene at position 444 (and keep the old '444' as '445') a hybrid norn can either get my new 444 gene or the default 444 one, right?

So in conclusion I should put all new genes at the end of the genome?

In the hair appearance gene thread someone mentioned that norns inherit whole chunks of organs.. is that true? But that would mean if I wanted a new gene to go with another 'default' gene I just would have to put them in the same organ, not in the end of the genome and hybrids would either have the one organ or the other, missing no important genes.. I'm confused

Lastly, I may be a bit out of creativity with this. I'm giving my breed a 'mystery chemical', initial concentration is set and halflive to 0 for the start.
Now I don't want them to have the same level of the chemical for their whole life, what would be the easiest way to get some fluctuation into it (setting the halflive right in the process, of course)?