Will my child have TS?

This is a tough question to answer with any trait, but a semi dominant one that is often not noticed and has such a wide phenotype (way of showing itself) is pretty tough. Basically, there is no sure way of telling, there are so many variables. Even if we could walk into a store and get our genome mapped, we could still only give probabilities. And there’s always the chance of a random mutation...they happen all the time. And even if your son does have a GTS gene, there’s only a 2.2% chance he’ll have Tourette syndrome.

I can give some Punnett square examples of the odds that a your child will have Tourette’s, but you need to know your own and your partner’s genetic makeup.
The tried and tested method for doing this in genetics is not easily applicable to humans. The way to discover a being’s genotype is you breed it with its own siblings or its parents. But, as I said, you can’t go breeding people with their sisters and anyway, there just isn’t enough of us. So the best we can do for higher life forms is a pedigree.

A Tourette pedigree should take into account all of the phenotypes of GTS.

That is: Tourette Syndrome, OCD, Chronic Tic Syndrome, Transient tic syndrome.

A pedigree is basically a family tree – using squares for males and circles for females. You should then colour in all the shapes of people who showed the GTS phenotype.  The example below is my family tree.

We saw on the Genetics of TS page that the penetrance for the GTS gene is 1.000, 1.000 and 0.002 for males.
That is, a male with two GTS genes (GTSGTS) has a 100% chance of showing the GTS characteristics.
A male with one GTS gene (GTSgts) has a 100% chance of showing the GTS characteristics.
A male with no GTS genes has an 0.2% chance of showing the GTS characteristic.

Because there is a 100% chance (certainty) of a male with at least one GTS gene showing the tic phenotype, assume that if a male person does not/did not show the phenotype for GTS then they don’t have the gene.

The penetrance for females is 0.709, 0.709 and 0.000.
That is, there is a 70.9% chance that the female will show GTS characteristic if she has two GTS genes.
Likewise, if she has 1 GTS gene (GTSgts) she has a 70.9% chance of showing the characteristics. But you'll notice that she also has a roughly 30% chance of having more than one GTS gene, but not showing the characteristics. So, in your pedigree you should keep in mind that any females who do not appear to show the GTS gene may still pass it on. People who can pass a gene on to their offspring, but are not affected themselves are called carriers.

As my mother's family doesn't show any signs of the GTS spectrum it is safe to assume she doesn't possess the gene.  But my father does have the gene, so I am almost certainly GTSgts.

From that you should be able to work out if you’ve gotten (or are likely to have gotten) the GTS gene from either or both of your parents. If you don’t seem to have gotten a GTS gene from either parent but you have Tourette syndrome; don’t panic. It doesn’t mean your father is actually the milkman, it’s either a random mutation, you’re one of the 0.2% of males who get Tourette syndrome without the gene, or the signs are so subtle you’ve missed them. Once you’ve found out how many GTS genes you have, and how many your partner has, use the following table to find the likelihood of your child having Tourette’s. Or you can just look it up for interest’s sake.

Click here to see how I got these figures 

References

Penetrance (taken from Comings et al. 1984) for GTS
100% for GTSGTS homozygotes MALE
100% for GTSgts heterozygotes MALE
0.2% for gtsgts homozygotes MALE
70.9% for GTSGTS homozygotes FEMALE
70.9% for GTSgts heterozygotes FEMALE
0.00% for gtsgts homozygotes FEMALE

Penetrance (taken from Walkup et al. 1996) for Tourette syndrome
100% for GTSGTS homozygotes
2.2% for heterozygous MALE
0.3% for heterozygous FEMALE
Assume 0% for heterozygous gtsgts

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