A team of UK scientists have undertaken what is described as the biggest DNA analysis of whole human genomes.
Their findings will mark the biggest single leap in the understanding of how genes dictate our propensity to get ill, opening the door to a new wave of tailored therapies and tests to predict the likelihood that people will develop a serious illness.
A coalition of 50 groups, including 24 leading human geneticists, has completed a study which in the course of the next year will reveal genetic signposts for heart disease, diabetes, bipolar disorder and hypertension.
Over the coming months, the consortium will release similarly detailed findings in major journals for eight common illnesses. The researchers have analysed 19,000 DNA samples, which include 2,000 for people who suffer from each of the diseases and a control group of 3,000.
Separately, they are examining 15,000 markers for genetic variations relating to another four diseases - breast cancer, autoimmune thyroid disease, multiple sclerosis, and ankylosing spondylitis, a chronic swelling of the joints.
Cardiff psychiatrists taking part will reveal genetic variations which dictate the probability a person will develop bipolar disorder. "The genes will tell us which biological systems are involved and allow us to predict who is at risk," said Nick Craddock, leading the bipolar disorder research.
Mark Walport, director of the Wellcome Trust in the UK, which sponsors the programme, said: "This is the first time that anyone has been able to look at genetic variations at this scale - results for some other diseases are as important as those for obesity."
Genetic tests are currently available for specific genetic disorders, but the Wellcome Trust Case Control Consortium is looking at the more mild and multiple genetic variations which combine with environmental factors to give a person an increased risk of developing a disease.
Earlier this week Exeter and Oxford Universities revealed a genetic variation responsible for obesity carried by one in six people.
Those carrying one copy of the "fat" FTO gene had a 30 per cent increased risk of being obese compared to a person with no copies of that version, scientists found.
Those carrying two copies of the variant had a 70% increased risk of being obese, and were on average three kilos heavier than a similar person with no copies.
Peter Donnelly, Wellcome Trust consortium chair, said: "We are trying to identify the genetic factors which make some people more likely to develop a disease than others. We know genetics play a part but we haven't found the parts of the genome involved."
However, some scientists have pointed out that there are other, as yet undiscovered, genes involved in obesity, casting doubt on the usefulness of a test aimed at FTO alone.
Obesity is a complex disorder and there is no evidence from this research to suggest that carriers of the gene will not successfully lose weight by decreasing their energy intake and becoming more active, they have said.
They have further warned that genetic testing could be problematic where genes are identified before therapies exist, meaning that people could be told that they have a risk of developing an illness, but with no options to treat it.
There are also fears that testing will fuel the fledgling commercial market for genetic testing and that people will not receive the counselling required to react to a positive result for an illness.