In addition, they said other genetic and environmental factors play their part, shown by the fact that thin people can also carry the genetic mutations.

BBC: NEWS | Health | Faulty gene link to obesity risk

However, there are a few noteworthy exceptions - for example, age-related macular degeneration - where the genetic risks explain a substantial proportion of the disease and which , when combined with other non-genetic factors, might be able to accurately identify individuals who will develop the disease in future.

BBC: Has decoding the genome lived up to hype?

Professor Meadow's figure on the likelihood of cot deaths was disputed by other experts who argued genetic and environmental factors made the odds of a second cot death in the same family closer to 200 to one.

BBC: NEWS | Health | Experts buoyed by Meadow ruling

But it added there appeared to be a link but the vaccine "is likely to have interacted with genetic or environmental factors which might raise the risk of narcolepsy, and that other factors may have contributed to the results".

BBC: Narcolepsy link to swine flu vaccine investigated