A woman is suing a London NHS trust for not revealing her father had been diagnosed with Huntington's disease before she had her own child.
She only discovered he carried the gene for the degenerative, incurable brain disorder after her daughter was born.
The woman then found out she too carried the faulty gene, meaning her daughter has a 50% chance of having it.
The NHS said the case raised competing duty of care and duty of confidentiality issues.
The claimant is known as ABC in order to protect the identity of her daughter, who is now nine.
The facts of the case are tragic. In 2007, ABC's father shot and killed her mother.
He was convicted of manslaughter on the grounds of diminished responsibility and detained under the Mental Health Act.
It was suspected that he might be suffering from Huntington's disease, a fatal neurological condition which progressively destroys brain cells.
Huntington's affects movement, cognition and typically causes altered behaviour and often aggression.
When his diagnosis was confirmed in 2009 by doctors at St George's NHS Trust, ABC's father made it clear he did not want his daughter informed. She had told him she was pregnant.
He told doctors he feared she might kill herself or have an abortion.
Four months after her daughter was born, ABC was accidentally informed about her father's condition.
She was tested and found that she had inherited the Huntington's gene, which means she will eventually develop the disease.
Her daughter has not been tested, but will have a 50:50 chance of inheriting it.
The woman alleges that St George's NHS Trust owed her a duty of care to tell her of her father's diagnosis, given that doctors there knew she was pregnant.
ABC says she would immediately have had a genetic test and would have terminated the pregnancy, rather than allowing her daughter to run the risk of inheriting the disease or having to look after a seriously ill parent.
At the time, ABC and her father were undergoing family therapy organised by the NHS, so she argues that there was an an obligation to protect her psychological or physical well-being.
This is a cornerstone of the doctor/patient relationship, but it is not absolute.
Disclosure of personal information, without the consent of a patient, may be justified to prevent exposing others to a risk of death or serious harm.
This case was first argued at the High Court in 2015 when a judge ruled that a full hearing should not go ahead.
The judgement said there was "no reasonably arguable duty of care" owed to ABC.
But in 2017, the Court of Appeal reversed that decision and said the case should go to trial.
If ABC wins the case, it would trigger a major shift in the rules governing patient confidentiality, and raise questions over the potential duty of care owed to family members following genetic testing.
Emily Jackson, a law professor at the London School of Economics, said: "If a patient doesn't want her doctor to tell her children about her terminal diagnosis, for example, or her HIV status, then it goes without saying that the doctor should respect the patient's confidence.
"The complicating factor with a genetic diagnosis is that it isn't just information about the individual patient, but it also reveals that his or her relatives are at risk.
"In such circumstances, and where the relative could act upon that information, should the doctor's duty be extended to the patient's close family members?"
A spokesperson for St George's Healthcare NHS Trust said: "This case raises complex and sensitive issues in respect of the competing interests between the duty of care and the duty of confidentiality.
"It will be for the court to adjudicate on those issues during the trial."