| MadSci Network: Medicine |
Cystic fibrosis is the most common autosomal recessive disease in Caucasians, with an incidence of about 1 in 2,500 live births. A genetic defect gives rise to a disorder of ion transport across cell membranes, which leads to damage to the cells lining the respiratory system, and in turn to repeated infections, which ultimately lead to progressive destruction of lung function, with inflammation and scarring. The other main organs to suffer damage are the pancreas (leading to malabsorption of foods and vitamins, and increases the likelihood of developing diabetes), and the reproductive system (leading to infertility). Being a genetic defect means that you are born with the disease, although lung function is normal at birth.
If your friend's siblings have the disease, it means that, assuming their parents do not have any symptoms of the disease, both of their parents must be carriers of the defective gene: they have one normal copy and one defective copy. The single normal copy is sufficient to keep the parents from getting cystic fibrosis. However, each of their children has a 1 in 4 chance of developing the disease (50% of getting the defective gene from the father times a 50% change of getting the defective gene from their mother -- .5 x .5 = .25). Keep in mind that this rate of probablilty only applies for families in which both the mother and father are carriers for the disease. I would think that if your friend is in his (or her) teens and has no symptoms by now, the likelihood of developing problems is remote.