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(To preface this, I've 0 knowledge of medicine)

If diseases such as Huntington's are hereditary and you can't develop/catch them later in life, how did the original person in the family bloodline get it? Because if their parents didn't have it and, therefore they weren't born with it, surely that means that they must've developed or acquired later it in life meaning that you either could've developed way back when or can still develop it now?

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    Welcome James. We like people to show their researches when posting. Have you looked at the age range that Huntington's normally kicks-in at? Compare that with the range of ages it can kick-in at. Are you sure the people in your example lived long enough to exhibit symptoms? (life expectancy figures). Was diagnosis common at that time. It's a question that you should look into the background detail yourself, then edit your question accordingly. Please take our tour and refer to the help center particularly the How to Ask section.
    – Jiminy Cricket.
    Sep 18, 2022 at 1:54
  • You don't "acquire" genetic diseases later in life.They may show up later in life, but they are "acquired" well before birth. If they are inherited, they come from the egg or sperm. Read about Huntington's Disease; a lot has been written about the genetics of it.
    – anongoodnurse
    Sep 18, 2022 at 2:06
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    OP is right -- there had to have been patient zero. P0 was either the result of a random genetic mutation somewhere along their ancestral line, or they acquired a gene from their pre-human ancestors in which the gene was perhaps harmless or even beneficial to those species. We've got a lot of genes from slithery things, after all.
    – Carey Gregory ♦
    Sep 18, 2022 at 3:09
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    @anongoodnurse Minor nitpick: In genetics, you would usually say that P0 has a de-novo mutation, opposed to P0 "inherited" it from the egg or the sperm. The haploid gametes constitute P0, P0 does not "inherit" mutations from itself or it's halves (oocyte/spermatocyte).
    – Narusan
    Sep 26, 2022 at 19:53
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    @Narusan - I understand better how poorly I phrased that. Thank you for the correction/clarification. You're right (as usual!) I'm glad you're still here. :)
    – anongoodnurse
    Sep 27, 2022 at 1:38

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You are almost spot on!

Children usually do not have exactly the same DNA as their parents, because it is evolutionarily wise to allow some new traits to emerge.

So, there can be spontaneous mutations in the DNA of a child that their parents did not have. In the case of Huntington‘s disease, approximately 10% of cases are actually such new („de novo“) mutations that are not inherited. They are uncommon, but not extremely rare.

These changes in the DNA happen to the sperm and the egg which become you, so once you are born (or actually once the zygote has been formed), you either have the mutation or you don’t.

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