Genetic talk is more of a guideline then a rule, although most follow how they are supposed to, even when something unexpected pops up.
Regards breeding; Although you said you're not going to do it anyway.
Not only that there are hundreds of rats being pts or released into a field because rescues are too full up to take anymore in (I'm not exgatuating here)
but there is the unknown genetics that are lurking in the background of an apparant healthy rat.
For example, I have lost two of my rats recently to cancer/facical tumours at only 1 years old. They were perfectly healthy up til then. And it slowly ate away their faces and spread into thier ears and throats until one had to be pts or he would've have died anyway while the other died in his sleep. It happened one after the other a month apart, so just as I got over one death the other started the same way shortly after. They were rescues just sadly bred for the sake of money. And most of their family has had cancer or resp problems. Could you live with yourself if you were the one who brought these rats into the world? Caused these problems because you wanted to breed your fav rat?
I also have rats with resportory problems for most of their older lives. They're only over 1 year and the other 2 just under.
I also have one boy who had to be neutered at 1 year and a bit because he became so aggressive that he had to be seperated until three weeks after being neutered, thankfully he's back with them, but is riddled with tumours. I mean there are loads of them on his back end and on his legs, they're small so far. He's not even one and a half. One sibling died before reaching a year, another has tumours too. donno about the others though, but from what I hear they have resp problems like my other one. Could you feel comfortable that if you breed this is what could happen? Aggression so bad that they have to be neutered or live the rest of their lives alone? Or riddled with tumours before they're 1 1/2 years?
So there are other factors to consider then just how many homes you're taking away from rescues, but what lives you would be creating too. Unless their reletive's health are known you are playing russian roulette with lives. Sometimes it's fine, other times it's very, very bad. With monatored lines it can cut down the risk of genetic diseases popping up. And people who usually go for well bred lines are either always going to get rats from well bred lines or nothing, or have recently lost rats to terrible illnesses that cut their lives short and need at least one rat that won't suffer from resp problems.