Why does a person need to have two copies of the cystic fibrosis allele to have cystic fibrosis?

Question: Why does a person need to have two copies of the cystic fibrosis allele to have cystic fibrosis?
- Keywords:
  allele,
  copy,
  cystic fibrosis
Asked by Esha to Barbara, Matt, Ravinder, Sophie, Tristan on 7 Mar 2015.
- Matthew Moore answered on 7 Mar 2015:
  
  Cystic fibrosis is what’s known as a recessive disorder, in this example what this actually means is that there is a protein (CFTR) which is encoded by the CFTR gene. When the CFTR gene is mutated the CFTR protein doesn’t work properly. We have two copies of every gene, so if a normal copy and a mutated copy are inherited the normal copy will produce the functioning protein and there will be no cystic fibrosis, the condition caused by having a broken CFTR protein.
  
  This means that two mutated versions must be inherited. This also means that people can be ‘carriers’ of the mutated version, where they have one normal gene and one mutated.

Question: Why does a person need to have two copies of the cystic fibrosis allele to have cystic fibrosis?