7. The Statistics of Tourette's Syndrome
As discussed earlier, males and females have different chances of inheriting Tourette's Syndrome. Firstly, the definition of penetrance needs to be introduced. Penetrance is the statistical probability of expressing a characteristic phenotype. It may be expressed as a probability (0-1) or as a percentage (0-100%).
A male having either of the genotypes AA or Aa will definitely show one of the three conditions (TS, OCD, CTS). In these circumstances the penetrance is said to be 1 or 100%. A male having an aa genotype has a penetrance of 0.2%.
A female which carries either the AA or Aa genotype has a penetrance of 70.9%. If the genotype of the female is aa then the penetrance is 0%. It is statistically impossible for a female of this phenotype to have any of the three associated disorders.
Note: There are exceptions! A statistically impossible situation as stated here actually has some chance of occurring. This is due to the genetics being far more complicated than that explained here.
More detailed information on the genetics of Tourette's Syndrome (including the chances of a child inheriting the disease) can be found at: