**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%).

**Males:**

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%.

**Females: **

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:*