Huntington's Disease is caused by a flaw in the DNA make-up of a person. DNA is what makes you the physical and emotional personal that you are, and when the DNA is damaged then, predictably, unpredicatable results may occur.
Huntington's disease is one of those "unpredictable results" caused by flawed DNA. In 1983 the disease was isolated to chromosome 4 (we all have 46 chromosomes). Then in 1993 the gene, which when damaged causes Huntington's disease, was found - which is now referred to as the HTT gene (or the huntingtin gene).
Scientists describe DNA as a series of four proteins that bind together in a long strand of DNA. Three of those proteins make up the CAG. Cytosine, Adenine, Guanine. A single "repeat" consists of a single sequence of Cytosine, Adenine, and Guanine linked to each other in sequence on the DNA strand, i.e. C-A-G. Two "repeats" would be C-A-G-C-A-G. If you have more than 36 repeats on the HTT gene then you will eventually develop symptoms of Huntington's disease.
The genetic test for Huntington's disease is a measure of the number of CAG repeats found on the HTT gene.
When a child inherits the HTT gene from the father it is very possible that, because of the instability of a damaged gene, the CAG repeat will expand. This is much less likely when the parent having Huntington's Disease is the mother.