why Huntington disease is rare ?

Answer 1

The previous answer is partly wrong. Huntington's is actually dominant. So you would presume that is would be common but isn't. This is because homozygous dominant zygotes CANNOT divide. They are complete non-starters.

For this reason, you can deduce that Huntington's sufferers are heterozygous. This means that they can have children that are 'Normal' because they carry the recessive gene too.

The previous answerer is right in saying that Huntington's sufferers do present symptoms fairly lately (in 40's or so, sometimes earlier), so they in most case already have had children. This is generally how the disease continues to exist, but in low incidence (as you would know you are at risk if your parent develops it and would seek genetic counselling.

Answer 2

because it is recessive genetic inherited and also not commonly seen until later on in life...not to mention it deteriorates your brain pretty fast so the victims die pretty fast....hope you don't have it