The cystic fibrosis gene is inherited from both parents and it can skip generations. It is autosomal recessive meaning that it occurs equally in males and females. In Australia, one in 2500 babies are born with cystic fibrosis, that is one every four days. On average one in 25 people carry the cystic fibrosis gene and most are unaware that they are carriers. Because carriers of cystic fibrosis are unaffected and therefore show no symptoms, it is hard for them to appreciate that cystic fibrosis may be a real risk.
Any one of us could be a carrier and not know it. There are about one million unaware carriers in Australia. In Tasmania, one in 20 people carry the cystic fibrosis gene. This is the second highest rate in the world behind Ireland.
When an infant received the gene from one parent, that are not born with cystic fibrosis but are a lifelong symptomless carrier, just like one of their parents.
Since 1986, all newborns have been subject to a heel prick test. This test is used to screen for several conditions, one of which is an indicator for cystic fibrosis. Diagnosis may also result after a baby is born with obvious cystic fibrosis symptoms such a bowel blockage or failure to thrive. The heel prick test does not detect cystic fibrosis in everyone, and the sweat test continues to be the definitive test, as high levels of salt in perspiration is extremely common amongst those with cystic fibrosis.
Cystic fibrosis came into the human genome about 5 000 years ago and it has survived and spread. Today there is thought to be almost 2 000 mutations that lead to cystic fibrosis, some of which are particular to individual families. At the other extreme, one mutation, Delta F508, is present in some 72% of all cases worldwide.
Carriers are symptomless and live normal lives. They are overwhelmingly unaware of their cystic fibrosis carrier status. Yet they have every chance of passing on the gene to the next generation and that generation to the next and so on – it is only a matter of time before a member of the carrier’s family is born with cystic fibrosis.
Two carrier parents have a 25% chance of having a child with cystic fibrosis with each pregnancy.
· 1 in 4 chance the child will have cystic fibrosis or a 1 in 4 chance the child will not have cystic fibrosis (will not have a copy of or be a carrier of the cystic fibrosis gene)
· 2 in 4 chance the child will be a carrier of cystic fibrosis (having one copy of the cystic fibrosis gene)