>like in the case of mental illnesses such as depression or Schizophrenia, scientists insist on looking for genes or even GWAS correlations and just aren't finding anything
Source? Wikipedia says:
>A central point of debate on GWA studies has been that most of the SNP variations found by GWA studies are associated with only a small increased risk of the disease, and have only a small predictive value. The median odds ratio is 1.33 per risk-SNP, with only a few showing odds ratios above 3.0.[1][46] These magnitudes are considered small because they do not explain much of the heritable variation. This heritable variation is estimated from heritability studies based on monozygotic twins.[47] For example, it is known that 40% of variance in depression can be explained by hereditary differences, but GWA studies only account for a minority of this variance.[47]
which admittedly isn't airtight proof for "depression is 100% genetic!!1", but isn't exactly "just aren't finding anything" either.
It is a bit more nuanced than that. An incorrect unstated assumption here is that schizophrenia or depression is a singular disease rather than a group of observable symptoms where any two cases may or may not have the same underlying cause.
There are certain types of schizophrenia that are highly heritable and others that are more sporadic. Highly pathogenic variants tend to be very rare because those regions of the genome are under evolutionary constraint.
GWAS are used is to figure out which genes may be involved in the pathology. So we are essentially looking at rare cases of inherited disease to figure out which genes might play a role in more common non inherited versions.
Source? Wikipedia says:
>A central point of debate on GWA studies has been that most of the SNP variations found by GWA studies are associated with only a small increased risk of the disease, and have only a small predictive value. The median odds ratio is 1.33 per risk-SNP, with only a few showing odds ratios above 3.0.[1][46] These magnitudes are considered small because they do not explain much of the heritable variation. This heritable variation is estimated from heritability studies based on monozygotic twins.[47] For example, it is known that 40% of variance in depression can be explained by hereditary differences, but GWA studies only account for a minority of this variance.[47]
which admittedly isn't airtight proof for "depression is 100% genetic!!1", but isn't exactly "just aren't finding anything" either.