Tourette syndrome is a multigene disorder. Unfortunately for all the female Touretters out there, this causes a slight disadvantage compared to the men.
Females with Tourette syndrome are more likely to have children with the condition than males with Tourette syndrome.
Why is this true?
Surely the genes that give a man TS are the same ones that give women the
condition.
Well, no actually.
It has been mentioned before on this site that men are more likely to get Tourette syndrome than females. This is not because the gene is sex linked, it has nothing to do with the Y chromosome that makes a man a man. Tourette syndrome is autosomal - it only appears on the non-sex chromosomes.
It appears men have a much higher rate of Tourette syndrome because of something that is common to every man - androgen hormones. Androgens are what cause the deeper voice, hair growth etc. that defines a man. And, not surprisingly, men have a much higher level of androgens than women do.
It was found by Leckman and Scahill in 1990 found that anabolic steroids (which is basically androgen in a pill) made Tourette tics worse. Since then, strong links between androgens and Tourette syndrome have been made. Men are more likely to get Tourette syndrome because of their hormones.
But what has that got to do with females and reproduction? Let me bring back the normal distribution graph:
Those values to the right hand side are the people who have a large number of Tourette syndrome genes. Those to the left have few or no Tourette syndrome genes. Notice there are less people at the extreme ends of this graph than in the middle.
Now, we've said that men have an androgen boost to their Tourette syndrome. What this means is that it is easier for them to get Tourette syndrome without as many genes as the graph would suggest. Women do not have this "advantage" and must have a full complement of Tourette genes before it can be expressed. So If I draw the thresholds for males and females actually showing Tourette symptoms, it would look like this:
To the right of the blue line, men will show Tourette's. To the right of the pink line, women will show Tourette's. This shows what we already know, more men have Tourette's than women do.
What all this means in a reproduction sense is women Touretters will generally have more Tourette genes than a man would. This means they have more genes to pass down to their children. When a child receives a large number of Tourette genes, and this takes them over the threshold, they will show Tourette's.
So this does unfortunately mean that women are more likely to have children with Tourette syndrome than men are.