Genetic testing can be used to confirm a physical diagnosis if there is no family history of HD. Medical diagnosis of the onset of huntington’s disease can be made following the appearance of physical symptoms specific to the disease. Even before the onset of symptoms, genetic testing can confirm if an individual or embryo carries an expanded copy of the trinucleotide repeat in the HTT gene that causes the disease.
Genes are arranged in precise locations along 23 rod-like pairs of chromosomes. One chromosome from each pair comes from an individual’s mother, the other from the father. Each half of a chromosome pair is similar to the other, except for one pair, which determines the sex of the individual. This pair has two X chromosomes in females and one X and one Y chromosome in males. The gene that produces HD lies on chromosome 4, one of the 22 non-sex-linked, or “autosomal,” pairs of chromosomes, placing men and women at equal risk of acquiring the disease.