Huntingdon's Chorea is an inherited condition(..)
I've never questioned the
medical relevance of family and hereditary history.
It's a matter of degrees. If the family history may impact on your ability to perform your duties as a pilot then it is relevant to the application and the CAA medical department have right to know. It's a matter of public safety where the needs of the public outweigh the individual's right to confidentiality
Now you are talking about the confidentiality of an individual (the family of the applicant) that might not fly and never applied for a medical certificate.
As mentioned, the applicant can only decide to give up confidentiality voluntarily (e.g. applying for a medical) regarding someone he can decide about. He can only decide about himself. That's why neither he or the AME can access medical records regarding a family member. Having said that, common sense says, as Bob the Doc states, that the AME can make a better medical assessment if family history is known.
If public safety never is to be compromised we can't live in what we define as a "civilised society". Why do you think that the police can't detain you for more than a certain period of time without a court proceeding? If the police thinks you are a hazard to the public safety, isn't it safer for the general public to just throw the key away?
Anyhow, isn't it possible to detect something like Huntington's Chorea by analyzing the blood of the applicant? I read that HIV is disqualifying for single pilot ops. HIV can be detected by analyzing the blood but those tests are normally not performed as part of aviation medicals. Does that mean that pilots are encouraged to have HIV tests done if they know that a positive result will give them an OML restriction? It's probably cost prohibitive to perform HIV and other tests on all applicants. I guess that it's a compromise between public safety and the integrety and economical burdan of the applicant, where the latter outweighed in accordance with the rulemakers.