Heterozygotes will pass the defective gene from one generation to the next. However, if carriers marry, the offspring will have a 1 in 4 chance of being affected and homozygous, a 1 in 2 chance of being a carrier and a 1in 4 chance of being genetically normal. Consanguinity (relationship by blood) increases the risk � in first-cousin marriages for example, there is a 3% theoretical increased risk of having an abnormal child.
In northern Europe, somewhere between 1in 22 and 1in 25 of us are carriers for cystic fibrosis with a sufferer in about 1 in 2000 live births.
Cystic fibrosis is caused by (hold on, it�s going to be very technical here!) a malfunctioning gene called the Cystic Fibrosis Transmembrane Conductance Regulator, usually abbreviated to CFTR (Thank, God!), which is 1480 amino acids long. This protein is a chloride channel protein and has a major role in maintaining secretions in airways and ducts in the body. It does this by hindering sodium uptake whilst excreting chloride ions. All organs that rely on mucus are affected by CFTR malfunction including the lungs, skin, pancreas and liver etc.
One of the problems with Cystic fibrosis is that allelic heterogeneity occurs. This is where different forms of mutation occur within the same gene. At the last count, there were over a 1250 such mutations. These different mutations result in different clinical presentations of patients, which is why doctors never see cystic fibrosis patients with identical symptoms.