Take height, for example. We know that tall parents have tall kids, but if you start looking for specific segments of DNA, you'll find over a thousand, none of which contribute more than a millimeter. The top 180 genetic markers still only account for 10 to 12 percent of the height variation from person to person. The rest of the story refuses to be pinned down. Brilliant people have spent years staring at these numbers, unable to sort them into any kind of rational order, even though to the untrained eye, there seems to be no mystery at all. Tall mom, tall dad: tall kid. But the closer we look, the less we understand.
Misha Angrist defends genome testing against Brandom's critique:
If we’re talking about hereditary breast cancer, an autosomal dominant condition, then a single bad copy of the BRCA1 or BRCA2 genes would mean something like an 80% lifetime risk of developing breast and/or ovarian cancer in female carriers. 23andMe tests for the three most common hereditary breast cancer mutations in Ashkenazi Jews. As it happens, I am of Ashkenazi descent and my Mom had early-onset breast cancer (she is alive and well and is wondering why you never call). Thus, when I learned from 23andMe and DNA Direct that I did not carry any of those mutations and therefore could not transmit them to my daughters, I was relieved. Fucking right I was! Say what you will about consumer genomics, but hereditary breast cancer risk information is not useless.
Razib Kahn seconds Angrist:
A non-trivial minority of people do receive actionable information from personal genomic results. By and large I am skeptical of individual risk prediction, and I communicate that skepticism to friends. But in one case a friend ended up with a large effect macular degeneration mutation. Before he had signed up for testing I told him to sleep through the risk prediction part. I don’t do that now. Chances are there won’t be any surprises. But some serious information will be received by 1 in 10 to 1 in 100.