Family gene that can bring tragedy

One of the most frightening aspects of cancer is its unpredictability. In most cases, it strikes out of the blue where there is no history and no apparent cause. But in one in ten cases, there is a family link, suggesting a genetic predispositon.

Professor Mel Greaves, of the Institute of Cancer Research, said: "We have two families under treatment at the Royal Marsden hospital with leukaemia in three generations. If it were in two, it could be coincidence but when it is in three that is stretching it. Given the genetic predisposition in common cancers such as breast and colon it would be surprising if leukaemia were an exception. The difference with leukaemia is that we don't know what the gene is."

The strength of the genetic predisposition can be gauged from the number of cases within the broader family. If there are many siblings in each generation and only one or two are affected it is likely to be weak requiring an environmental trigger to cause the cancer. But some genes are so strongly predisposing, there is no need for an environmental trigger.

A further clue comes from the age at diagnosis. If it is younger in each succeeding generation, as in the Merritt case, it is suggestive of inherited predisposition.