>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
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.
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.