1. Out of curiosity. Kidney disease heritability. "We report heritability estimates of the CKD-defining traits eGFR (44%), UAE (20%), and UACR (19%), for the kidney biomarkers serum urea (31%), Scr (37%), and uric acid (48%), and for the serum electrolytes potassium (28%), calcium (27%), and sodium (22%) " They checked and basically every kind of family member with the disease elevates your risk, even "genetically unrelated" ones like spouses (assortative mating or maybe shared environment). sciencedirect.com/scien… It doesn't seem to be impressively heritable, but there's obviously some genetic causes of this, whatever they might be exactly.
2. There are some single mutations that cause schizophrenia alone, or schizophrenia like traits at least. pnas.org/doi/abs/10.107… "A total of 48.2% of individuals with severe, extremely treatment-resistant schizophrenia carried at least one rare, damaging missense or loss-of-function variant in intolerant genes compared to 29.8% of typical schizophrenia individuals".
3. Family studies yielding estimate of heritability, shared environment etc. tell you where to look for causes. They give you the overall importance of some group of causes, but doesn't tell you what they are except in a broad sense. In other words, they tell you where to dig for more. They also give you a decent idea about what kinds of interventions are going to be effective. When heritability is very high, it means that genetic selection will be effective. It scales linearly with the square root of the variance. If "family environment" doesn't show up or is very minor, then trying to modify risk factors in that area will not be effective unless you somehow introduce a new kind of thing that doesn't already vary between families. emilkirkegaard.com/p/hi…
4. Assortative mating and measurement error are usually quite large downwards biases in family studies. This is particularly the case for self-report symptoms of mental health vs. more objective data (e.g. clinician ratings). I wrote a summary of such research. emilkirkegaard.com/p/he…
