I dont think it is all in genetics, I have a rir, a cucco maran, and a GLW who are fine, a blue cochin, buff orp, SLW, EE are all the same with the fat bums, feathers missing and waterbelly look all are from different hatches but all eat the same feed. I do think the maran might be getting it also
Is it possible that ascites is caused by the feed? I had one cochin with it, now I have like 4 total whose bums look like that. They are all a year this month I feed eggmaker crumbles.