With very large repeat counts, HD has full Penetrance and can occur under the age of 20, when it
is then referred to as juvenile HD, akinetic-rigid, or Westphal variant HD.
This causes the number of repeats to change in successive generations, such that an unaffected parent with an"intermediate" number of repeats(28-35),
or"reduced Penetrance"(36-40), may pass on a copy of the gene
with an increase in the number of repeats that produces fully penetrant HD.