"Right now, family history is the single most important genetic test that can be performed," says Dr Joann Boughman, Executive Vice -President of the American Society of Human Genetics.

This may hold major consequences for men in the cases of paternity suits. When 14-year-old DI (Donor Insemination) child Brittany Johnson fell ill, her parents used a private detective to track down her natural father, a court order to force him to testify and a million-dollar lawsuit to punish the sperm bank he donated to. Here's what every man should now about his father:

Has he or his parents had cancer, heart disease, diabetes, asthma, mental illness, stroke, kidney disease or alcoholism? If you discover a string of similar illnesses in your family, you get a better picture of your own risk. For example, scientists have found an artery-clogging gene that almost doubles the risk of laying down fatty deposits.

At what age? And how bad was it? If these diseases strike your family at a young age or in an aggressive form, then they may have strong "gene expression", which could spell trouble for you if you inherited those faulty genes. A study in the Journal of Urology found that men who have a family history of prostate cancer are more likely to develop the disease at a younger age and to die of it than men who don't.

Did any of those relatives also have lifestyle issues like smoking or obesity? While some genes might put you at risk of heart disease, maintaining a healthy lifestyle may prevent the gene ever being triggered.

Does Dad or Mum have the gene for sickle cell, Tay-Sachs or cystic fibrosis? If you carry one of these genes and your wife also does, there is a 100 per cent chance your children will get the disease if they inherit the gene from both parents.

Was he or his parents born with any birth defects? These can be a sign that something is off-kilter in the genetic make-up. Messed-up genes might be passed on - something to consider if you're going to have children.

By Sara Vigneri