The sentence is poorly written. Perhaps it's 26% of the only reason. "The word "sentence" is 20% of the first sentence in this post (1 in 5 words = 20%)."
I hope they mean "Heredity only explains 26% of the cases of CD in children."
This seems to be the difficult subject of heritability. It's not 26% of cases (I think), but 26% of the contribution. If identical twins are studied, you can see how much variation (on average) relates to inheritance and how much to environmental or random factors.