Genetic Cystic Fibrosis – An Overview

Cystic Fibrosis affects nearly 100,000 people worldwide and is the most common autosomal recessive inherited disease in Caucasian people. Genetic cystic fibrosis may be inherited from the parents, but only if both parents have the CF gene.

What is autosomal recessive?

As humans, we all have 22 pairs of non sex-linked chromosomes that give us particular characteristics, like hair and eye color, as well as 2 sex-linked chromosomes. One of each pair of these chromosomes comes from the mother and the other from the father. The 22 pairs of non sex-linked chromosomes are called autosomal, and can be passed on to either a baby boy or girl.

The cystic fibrosis defect is found in the 7th pair of the 22 non sex-linked chromosomes, and so it is autosomal. Because it is non sex-linked, it can occur in either males or females.

Both chromosomes in the pair must have the defective cystic fibrosis gene for it to be passed on to an offspring. This is called recessive. If only one of the pair of chromosomes is defective, that person will not have cystic fibrosis but they will be a carrier for the condition.

How does a child inherit cystic fibrosis?

Parents often find it difficult to understand why their child has been born with cystic fibrosis when no other family member has it. There can be several generations of people who are carriers in a family line, and no one knows about it. It is not until a baby is born with the disease that anyone is aware that there have been carriers in the family.

Because CF is recessive, both genes in the pair must be defective for a baby to be born with it. Both parents need to be carriers, or have the disease themselves, for their child to have cystic fibrosis. Statistics show that 4% of all Caucasian people are cystic fibrosis carriers, and CF is more prevalent among Caucasians than other races. Even if both parents are carriers, it doesn’t automatically mean their children will have cystic fibrosis. There is a 25% chance that they will pass on their defective gene; so, there is a one in four chance that their children will be born with CF.

What causes cystic fibrosis?

The gene responsible for CF is called the cystic fibrosis transmembrane regulator or CFTR. CF occurs when there are errors or mutations of this complex and large gene. There are more than 1,000 possible mutations of the CFTR gene which can cause CF.

When both copies in the pair of the CFTR gene have a mutation, the child will be born with CF. If the child inherits one normal and one mutated gene, she will be a carrier. If neither gene in the pair has mutations, the child will neither have CF nor be a carrier for the disease.

So, there is only one way that a child can be born with cystic fibrosis:

  • Gets a mutated CFTR from mother + mutated CFTR from father = has cystic fibrosis
  • Gets a mutated CFTR from mother + normal CFTR from father = is a carrier
  • Gets a normal CFTR from mother + mutated CFTR from father = is a carrier
  • Gets a normal CFTR from mother + normal CFTR from father = not a carrier and doesn’t have the disease.

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