It's getting late but for the record you can find a good study of genetics and religion in Do Genetic Factors Influence Religious Life? Findings from a Behavior Genetic Analysis of Twin Siblings. PSYDIR offers a good summary:
It's a fairly standard twin study. They took a sample of around 600 identical and non-identical twins from the National Survey of Midlife Development in the United States (MIDUS), and looked at a number of religious characteristics.
Basically, their analysis allows them to tease out the variations that are shared by identical twins but not by non-identical ones (genetic factors), by non-identical twins (family factors or shared environment), and that differed even among non-identical twins. This last factor was put down to the effects of external environment (i.e. things that happen in you life that aren't shared by your twin).
I've put the results in the graph. First off, look at childhood religiosity. The biggest factor is your family, and not your genetics. It's not until adulthood that the effects of genetics really start to shine through. No surprises there!
The 'salience', or importance of religion in your life is about one-quarter defined by genetics, as is your spirituality. The most important factor here, however, is the external environment. You get similar results for religious attendance.
When you get to more personal beliefs, the patterns start to shift. There are three factors that are about 40% driven by genetics, with your family upbringing having hardly any effect. These factors are: how often you turn to religion for guidance, whether or not you take the bible literally, and whether people should stick to one faith, or experiment with others (exclusivist beliefs).
It is true that there are tricky statistical issues with twin research and it is certainly possible that results like these will be overturned in the future. If that happens, however, it will be because of better twin/adoption and direct genetic studies. The type of evidence that Tyler cites is simply not capable of answering the fundamental questions that are being asked by this type of research. It is also true that these results are conditional on an environment, that is a time and place. (But that is the relevant measure for parenting today.)
I would also note that if you think the statistics get the numbers wrong you also have to deal with the fact that the patterns make sense. Parents have the biggest influence on childhood religiosity, non-shared environment has the biggest influence on attendance, genetics has the biggest influence on being "born-again." (Even the word suggests nature.) Bryan's book reviews a number of studies like this which are broadly similar.