Roger Ingham has sent me an interesting article that adds another twist to the sex ratio story. A study by Drayna, Kilshaw and Kelly claims to have found a difference in the sex ratio between patients with a family history and patients without a family history. They say: "One possibility suggested by these data is that roughly half of all cases of stuttering is due to inherited causes, while the other half are due to poorly understood but nongenetic factors. This hypothesis is consistent with the view that persistent stuttering of nongenetic origin is largely a male disorder and may be related to a greater ability of females to overcome childhood stuttering (Ambrose et al. 1997). Genetic stuttering, in contrast, affects males and females more equally, which has important implications for genetic studies of this disorder that exclude young children. In particular, genetic-linkage studies will be much less obscured by the distortion in sex ratio, since this distortion is largely a phenomenon of sporadic stuttering." (Source: Am. J. Hum. Genet. 65:1473–1475, 1999)
This research is quite intriguing, and opens up a pandora box of questions. Here is what I fished out of the box: Does this mean that there are subtypes of PDS (e.g. genetic or non-genetic) or is it all the same just caused by different factors (e.g. I can break your arm with a metal bar or you fall)? Is there a stuttering gene? Or many genes in combination? What is recovery rates of genetic vs non-genetic? Is their research correct? How accurate are family histories? Why does the sex ratio differ between genetic and non-genetic?
For those who are not geneticists. Nor am I one! Here is how I visualise genes. I see the brain/body as a big city with factories, highways, buildings, people, etc. Many different objects are interacting to create an emergent structure. Genes are the blueprints of the objects that make up the city. A gene tells an object how to manufacture another object, e.g. how to manufacture cars, a building, a tool that lets two objects interact. Lets take PDS. Naively, I view it as a city WITHOUT an underground/metro in the speech and language areas. As a result, information travels slower and might get stuck in traffic jams, with secondary consequences on life in that area. Now, there are two ways how this could have happened. First, a gene or many genes are missing or not working, i.e. the blueprints for crucial objects like underground trains are missing. But sometimes even if a gene is missing, you can still build the underground but it might be harder. For example, if there are no genes/blue prints for escalators, less people can travel at peak hours. Or a gene might be missing, but its functionality can be easily replaced by another one. Second, all genes are there but something went wrong during the construction. Maybe there was an earthquake, an economic recession, a political intrigue, etc. And thus, they never build an underground system. Finally, it might well be that both effects interact...
OK this is just a picture but I think it pretty much gets the essence, no?