6. The Genetics
The science behind Tourette's Syndrome is quite complicated and much research is still being done to ascertain its causes and possible methods of curing those affected by it. So far the disease has been characterised as a genetic disorder. This section is designed to give a brief but scientific understanding of genetics in relation to Tourette's Syndrome. It all begins with the genetic make up of an individual. Genes combined with the environment make us what we are. Together they affect our height, shape, colourings and many many other features that make us what we are.
Diagram Reference: http://au.geocities.com/jones_kacm/
Genes are sections of a huge molecule called DNA. To the left is a diagram showing how DNA molecules, "the building blocks of life" arrange themselves in almost every cell of an organism. The structure resembling an X in the diagram is a chromosome. It is comprised of DNA which is wrapped around itself in a helix and then wrapped around proteins called histones. Each gene will code for a particular trait within an organism. Each gene consists of two alleles. It is these alleles which are important in Tourette's Syndrome.
In most cases alleles are dominant or recessive compared to their partner allele. This can be represented as an upper (a dominant allele) or a lower (a recessive allele) case letter. It is now necessary to define two words. The phenotype and the genotype. The phenotype is a physical trait which an organism has. This is determined predominantly by its genotype. The genotype consists of the two alleles. In simple cases where only one gene governs a particular phenotype, only three genotypes are possible. Here the letters B and b will be used to represent the dominant and recessive alleles respectively. The possible genotypes are BB, Bb or bb. In an organism, it is normally the case that the dominant allele (if present) will be expressed in the phenotype. If only recessive alleles are present, then the phenotype of the recessive allele will be expressed.
Tourette's Syndrome is governed mainly by one gene which here shall be denoted as A or a representing the dominance of the alleles. So the possible combinations are AA, Aa or aa.
Tourette's Syndrome is said to be semi-dominant. This means that the presence of the dominant allele makes it likely, but not certain, that an individual will have Tourette's syndrome.
The reason for this arises due to the A gene being responsible for a variety of diseases. These are: Tourette's Syndrome (TS) Obsessive Compulsive Disorder (OCD) and Chronic Tic Disorder (CTD). Thus, if you have the A allele, you can be affected by TS, OCD or CTD. It also means a parent with OCD can give birth to a Tourette's child, or visa versa. So having the A allele does not necessarily mean that one will have Tourette's Syndrome.