Spinocerebellar ataxia is a hereditary disease. It genetically passed from affected parent to a child. This disease is autosomal dominant, meaning that approximately half of the children will inherit the disease if at least one of the parents is affected. This can be seen below:



Fig. 5: Autosomal inheritance of SCA

Source: http://depts.washington.edu/neurogen/downloads/ataxia.pdf

This can also be seen by drawing a dichotomous key (please click to zoom in):



Fig. 6: A dichotomous key showing the autosomal inheritance of SCA

Source: No source

Although there are many different types of spinocerebellar ataxia (more information about the different types of SCA here), each with their distinct characteristics, all types are caused by a mutation in a certain gene which controls activity in the brain.

Fig. 7 - CAG repeat expansion

Source: http://depts.washington.edu/neurogen/downloads/ataxia.pdf

One particular mutation occurs in SCA types 1, 2, 3, 6, 7, and 12 and it's called a CAG repeat expansion. CAG, a part of the DNA sequences that codes for the protein glutamine, is repeated more than is normal, which then affects the proteins which it codes for.

In SCA type 8 and 10, the CTG and ATTCT pattern is repeated too many times respectively, while in type 14, a different sort of mutation (point mutation) occurs in the PRKCG gene. A point mutation occurs when a single nucleotide base is altered, added, or removed in a normal DNA sequence.

Some individuals who inherit the SCA gene with a repeat expansion actually don't show any symptoms in their lifetime. This is known as non-penetrance and is a very rare phenomenon. Researchers are still trying to find the cause of this.