Constant linebreeding too close for too long wears down the immune system in ANY type of animal. I have seen it with dogs,horses,rabbits,goats, and now chickens.
I bought all my Marans chicks from several different breeders because of that. I do keep a couple of the lines pure but those that I found "weaker" from the start or that seemed to have more genetic defects were culled or I let them free range and do not use them for breeding.
As for the original question...Yes I find that in my experience that the ones with the heavier leg feathering were more succeptible to being sick. Particularly form one breeder. Maybe ours came from the same breeder. I had a similar issue last year when I purchased 30 chicks. They were shipped form Texas to Ohio and they all arrived alive but as I got to inspecting them I found droopy eyes,webbed feet,crooked toes, and missing digits. I can't even believe the guy shipped them like that. Healthwise
they seemed fine until they were off the brooder then they literaly dropped like flies, some without even showing signs of illness. After all was said and done I have only two hens left from that bunch. The breeder had told me that his were all original WJ and that he had carefully line bred them to keep them form being too close. I find that hard to believe now and will never buy from someone like that again. I also had to disinfect and fumigate my entire poultry room in my barn before incubating and brooding this year just incase...
Another line I have is a Davis/Presley based line and it is very hardy BUT there is also a lot of English influence in it. They are not as tall as my WJ c1 roo but they are stocky and utilize their food much better. I find that the roos are firendlier and they also lay a much darker egg. I am however finding that it is hard to keep the feathers in the legs but I have grown to like them clean legged. I think they look much neater and they hold up better as far as free ranging.
This might sound wierd to some but I think that when you turn the chicks out it stresses them and if they have been harboring any illness unnoticed they will show it once they are off lights. That is why I now add asprin to their water the day I move them out and give them some fresh veggies and vitimins. I also keep them in the same area as my younger brooding chicks so they can still have the comfort of the light without as much of the heat.
But I am a little nutty about things and perhaps I consider their feelings too much. I just know I wouldn't like to live under a bright red warm light for all my life then be put in a cold & dark possibly damp coop all in one day.
Time to check the chicks!