In my last post, I discussed the mathematical unlikelihood of polygenic contributions to a heritable trait or disorder. Let's say you simply refuse to give up the ship. You just want to believe that there is a magical cascade of interactions between tens of thousands of genes that confer the phenotype you are looking for. Shortly, I will have a question for you...
As an example, let's consider the commonly espoused idea that intelligence or IQ is a polygenic process. This model is getting a lot of play due to studies that purport to show several different genes linked to intelligence, using a "polygenic score" or similar device. As I've mentioned, genes used to derive a high polygenic score might have no actual connection to the trait in question, but for the sake of argument, let's assume that all of these, say, 30 genes, are related to IQ in some way. Here's the question: How? Genes make RNA, which makes protein enzymes that convey chemical processes. So what is it that these genes would be doing better in a person that has a high IQ, as they work together to convey intelligence? Try to think of even a hypothetical model in your head that has plausibility. I'm guessing you can't.
If someone is sitting at a desk taking an IQ test, are there some sort of genetic processes at work at that moment that make them think faster or smarter, or make them more attentive, or move the electrical impulses to "smarter" parts of the brain? I would think that such a process would be too slow and cumbersome. Another possibility is that these genes don't do anything at all, anymore.
To illustrate that, let's take a look at something completely different: Height. It is presumed that height has a genetic component and even I would be hard-pressed to dispute that, although there has been no specific "height genes" discovered to my knowledge to date. Like IQ, it generally presumed that height is polygenic (I actually doubt that to some extent. I think we are probably missing something obvious, but it works well with this example). Here's the deal, though. I am 6 feet tall and 55 years old. When I was 17, I was also 6 feet tall. Like most people, I haven't grown at all since my late teens. So unless there are genes that keep you at the same height, it is likely that whatever genes might be involved in our height phenotype have done all they are going to do for us and are effectively dead, unless they had some dual purpose we know nothing of.
Now let's get back to IQ. Generally, IQ is held to be relatively static, even from an early age. Another possibility is that the genes in question were involved in brain development and have since done their job and are now dead. Then, we no longer have to worry about the current genetic mechanisms, since none exist. This, however, would require some way of correlating something about a person's brain, or maybe their neurons or some other physical factor that would make them better thinkers. No such correlations have been found. Quite the contrary, there seems to be little difference between normal brains.
So where do we sit? An unknown polygenic mechanism that confers intelligence in some unspecified way either in real time or by setting up our brains in some other unknown fashion. Basically, we really don't know a thing about what gives one person a higher IQ than another. Thus, we don't have any actual evidence that genes play any role at all in IQ or how such a process would occur. All we have is an assumption due to the apparent heritability of IQ.
I believe all I've said above could also apply to Schizophrenia and other mental disorders or even personality traits, so I won't belabor the same points unless someone actually comments here and draws a distinction.
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