Cystic fibrosis

Cystic fibrosis is an inherited disorder of cell membranes that mainly affects the lungs and digestive system. They can become clogged with lots of thick, sticky mucus as too much is produced. Over many years, the lungs become increasingly damaged and may eventually stop working properly. A number of treatments are available to help reduce the problems caused by the condition, but unfortunately average life expectancy is reduced for people who have it.

It is caused by a faulty recessive allele on chromosome 7. To be born with cystic fibrosis, a child has to inherit two copies of this faulty gene - one from each of their parents. Their parents will not usually have the condition themselves, because they will only carry one faulty gene and one that works normally.

In the diagram below cystic fibrosis involves:

the recessive allele (lower case), which can be shown as f

the dominant allele (capital letter), which can be shown as F

An individual who is homozygous (ff) with the recessive allele will develop cystic fibrosis. Someone who is heterozygous (Ff) will be a carrier of the recessive allele, but will not develop cystic fibrosis and have no symptoms. Someone who is homozygous with the dominant allele (FF) will not develop cystic fibrosis, as you need two faulty alleles (ff) for the condition. In this combination, no faulty alleles are present.