I don't know, if they're dying suddenly at 3 weeks, I'd suspect cocci rather than a lethal gene. Now, the low, low hatch rate could be a lethal gene, or it could be a number of other fertility issues. Did you break the eggs that candled clear to see how many had started development and then died? Oops, not "candled clear", I meant, did you break the fertile eggs that didn't hatch? If they were all within a few days of each other when they dies, maybe a lethal gene, otherwise, I'd be thinking incubator issues.