What would happen if the mice escaped the facility, and bite other animals or humans (or breed with 'normal' mice)?
Cockroaches escaped: https://www.telegraph.co.uk/news/worldnews/asia/china/102648...
Cockroaches escaped: https://www.telegraph.co.uk/news/worldnews/asia/china/102648...
Genes don't typically transfer that way. Except for extraordinary circumstances where the disease is caused by some sort of virus, getting bit by someone with hemophilia or cystic fibrosis or down syndrome isn't going to give you the disease.
>or breed with 'normal' mice
It's not impossible for this to happen, but in general these mice have been bred for generations in a lab environment where they are provided with food and water without having to find it on their own. The world is already a harsh place for a normal mouse that is well adapted to it, so if you add in a "soft" mouse with some sort of debilitating genetic disorder, I wouldn't worry too much about it taking over the world.
not a biologist in any way, but i'm pretty sure an addition/modification of one trait to an existing genetic line doesn't immediately render null-and-void all existing fitness of that line, such that they are immediately so unfit as to not propigate further
That's true, but these lab lines generally are inbred for generations on purpose before the disease gene is even added, so even the "normal" lab mice without the disease are quite different than wild mice, and even quite different than other strains of lab mice.