Huntington's Disease, which was called Huntington's Chorea at the time this episode was filmed, is a rare genetic disorder (prevalance around 5 cases per 100,000 people worldwide). It is caused by an inherited flaw in the HTT gene, found on chromosome 4. This gene codes for the protein huntingtin, which seems to have many functions and interacts with many other proteins. The flaw is autosomal dominant, which means that an individual with a single flawed copy of the gene will most likely develop some form of the disease. A part of this gene is a trinucleotide repeat, specifically, cytosine-guanine-adenosine, or CAG, which codes for the amino acid glutamate. Two many repeated CAG groups cause cells to manufacture a non-functional or toxic form of huntingtin, which eventually kills the cells affected. Because neurons do not generally regenerate, the symptoms are chiefly neurological - a vaguely dance-like movement called chorea, eventually leading to paralysis, cognitive degradation, and death. There is, sadly, no cure, but there are drugs to help manage the symptoms. Most who will develop the disorder begin to show signs of it during their 30s or 40s, but there are cases in children and cases in the very old. The number of CAG repeats in the flawed gene seems to correlate inversely with the age of onset and the rate of progression.
The end of the episode left open the question of whether Maslow suffered from Huntington's Disease, or from a similar set of symptoms caused by the stress of working on a high profile project, coupled with the fear of developing the disease. Maslow's grandfather certainly died of the disorder, but his father's death was less clear - and that is crucial. Because the disease is autosomal dominant, if a parent never develops it, the thread is broken and none of that person's descendants will develop it (unless one of them marries someone else with a bad gene, of course, which would put their children at risk).