Launching a new book on genetics, the BMA said genetics underlined the importance of "blood ties" and showed that, despite increasing family breakdown and fragmentation, people do have moral obligations to their relatives.

However, it does not believe these can be enforced or regulated by others.

Its book, Human Genetics: Choice and Responsibility, says that doctors - while encouraging patients to take a test - should make "strong efforts" to persuade patients to share information obtained by genetic testing with their wider family.

And it says they could be justified in breaking confidentiality in exceptional cases to protect other members of the family from harm and suffering.

Dr Michael Wilks, chairman of the BMA's medical ethics committee, said: "For some people, knowledge brings peace of mind and the ability to face the future. Other people prefer to remain in ignorance of what destiny holds for them.

"But do any of us have the absolute right to decide whether to be tested or not, without considering the often vital implications of that decision for our families?"

Wider public debate

The book explores a number of ethical issues related to genetic diseases.

It gives a general introduction to the subject and covers issues including the medial and social uses of genetic information.

The BMA wants a wider public debate on the ethical impact of genetic testing, particularly concerning people's respsonsibilities to their families.

Its call follows a meeting of the government's independent advisory body on human genetics which looked at ways of getting information on genetics to the public in a digestible form so they could join in the debate.

The BMA says it backs testing, provided patients are fully informed - despite the fact that knowledge of a genetic disorder, such as a predisposition to breast cancer, could provoke depression and despair.

No designer babies

However, it opposes the idea of "designer babies", saying genetics should only be used to reduce suffering and impairment and not for "trivial reasons or as a means of satisfying parental desires for certain physical or enhancing characteristics".

It also says commercial screening services should only be available for conditions which have no major health implications.

The BMA is against parents testing young children for genetic diseases which they may develop later in life.

It says testing should be delayed until the child is old enough to make their own decision on testing and consider the full implications.

The number of genetic tests available is set to mushroom in the next century

It can see no reason for people with genetic diseases to be deterred from having families.

It says: "The BMA strongly rejects the view that people with genetic disease should be deterred from having children, nor should they have a greater burden or moral responsibility imposed on them than other people.

"Ideally, all intending parents will think carefully about their plans for a family and all patients' decisions, not just those in a genetic context, would take account of serious implications for others."

Paternity testing

Other issues covered by the book include the controversial issue of paternity testing.

Doctors are receiving more and more calls to be involved in battles over maintenance payments and custody hearings.

Men want tests to prove or disprove their paternity and eligibility to pay maintenance.

The BMA says doctors should not test children if they think it is not in the child's best interests.

It also calls for a radical review of the concept of risk-based insurance due to the growing number of genetic tests available.

There are concerns that evidence of positive tests will be used against individuals.