Previously, I have discussed the spectrum of GTS as a continuous one. I would like to briefly retract that theory so I may discuss how a multi gene condition like Tourette syndrome comes about.
While the "trigger gene" for these conditions (GTS) is a single gene at the locus (position) 11q36, Tourette syndrome is caused by many different genes. The idea of the trigger gene is that it is the best indicator out of all the genes as to whether a person will be affected by TS or not. As stated on the gene page, if a male possesses the GTS gene, he will have an 100% chance of having Tourette's, while a female has a 70.9% chance. These are pretty good figures, they give a clear idea of what you can expect. However, you may have noticed on that page that there is also a slim chance that males can show GTS symptoms without any GTS genes. This is because of multigenic inheritance.
If a condition is caused by only one gene, for example brown eyes, then it is easy to tell what phenotype (appearance) the person will have. Two dominant genes or one dominant and one recessive gene (AA or Aa) the dominant characteristic will show through, if two recessive genes (aa) then the recessive one will show through.
If however the condition is caused by two genes, it's not so straightforward. You need to know which are the most dominant or which have the most effect or whether on cancels the affect of the other or any number of other bits of information.
Often, though, you can assume that most of the genes are
either dominant or recessive and have an additive effect. An example of
this is a plant with variable stem length. This plant has 6cm stems if it is a
recessive homozygote - aabb
If the plant then has an A gene it becomes 2cm longer. So
a plant that is Aabb would be 8cm and a plant that is AAbb would be 10cm.
If this same plant has a B gene it is 1cm longer. So a
aaBb plant would be 7cm and a aaBB would be 8cm.
However, these two genes are additive, so an AaBb plant would
have an extra 2cm from the A and an extra 1cm form the B, so it would be
9cm. An AABb would be 11cm, AaBB would be 10cm and AABB would be
12cm.
No one knows (yet) exactly how many genes are involved in Tourette's syndrome. But it is certain there is more than two. So the genetic makeup of a TS patient may be: AAbBccDdEEFFgGHhIIjjKkLlmmNNOOPPqQrRSSTTUUvvWWXxYYZz
It is not known how much gene K contributes to Tourette syndrome, nor if having two TTs is worse than two NNs. However, what is known is that the more complex a genetic code, the more that the likely outcomes fall in to a regular pattern. This pattern is a called a normal distribution.
This picture (above) is of a normal distribution. What
this shows is the number of people in a chosen population plotted against their
genotype. Basically it shows that it is uncommon to have a totally
homozygous genotype (AABBCCDDEEFFGGHHIIJJKKLLMMNNOOPPQQRRSSTTUUVVWWXXYYZZ
or
aabbccddeeffgghhiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz)
But it is much more
common to have some combination of dominant and recessive genes.
This means, in relation to Tourette syndrome, that the spectrum of genes is continuous. But with a continuous variation on genes, this means that there is no specific combination which causes Tourette syndrome. So what you get is a normal distribution like the one below:
|
In this picture, |
As you can hopefully see, there is a specific cut off point in this spectrum where people are said to be affected by one or more of these conditions. It also shows that there is a proportion of the population that is very close to having a GTS condition and other parts that are at an almost anti-tourette syndrome extreme.
So Tourette syndrome and other GTS conditions are part of a continuous spectrum at the far end of normality. But in order to have a diagnosis of any sort, there must be discreet cut off points in the population. Otherwise, nearly everyone in the world would be a GTSer, but only 1 in 50 will actually have problems with it.
This is why we have the DSM and ICR definitions of conditions. They are designed to tell us where the cut off points are.