What is cystic fibrosis, and why is it caused by a recessive allele?
What is cystic fibrosis, and why is it caused by a recessive allele?
Answer
Cystic Fibrosis (CF) is a genetic disorder that primarily affects the lungs and digestive system, characterized by the production of thick, sticky mucus that can obstruct airways and lead to respiratory infections, as well as blockages in the pancreas that impair digestion. The condition is caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene, which is crucial for the regulation of salt and water transport across cell membranes.
Inheritance of Cystic Fibrosis
Cystic fibrosis is inherited in an autosomal recessive manner. This means that an individual must inherit two copies of the mutated CFTR gene (one from each parent) to develop the disorder. Here’s how this process works:
- Recessive Alleles: In autosomal recessive disorders, both alleles of a gene must be mutated for the phenotype (the observable characteristics) to manifest. Individuals with one normal allele and one mutated allele are considered carriers; they do not show symptoms of cystic fibrosis but can pass the mutated allele to their offspring.
- Carrier Parents: If both parents are carriers of the CFTR mutation, each child has:
- A 25% chance (1 in 4) of inheriting two mutated alleles and developing cystic fibrosis.
- A 50% chance (1 in 2) of being a carrier like their parents (inheriting one normal allele and one mutated allele).
- A 25% chance (1 in 4) of inheriting two normal alleles and neither having CF nor being a carrier.
- Mutation Types: There are over 1,800 known mutations in the CFTR gene that can lead to cystic fibrosis, affecting the function of the CFTR protein. These mutations can vary in severity, leading to different manifestations of the disease among individuals.
Mechanism of Disease
- The CFTR protein functions as a channel that transports chloride ions across epithelial cell membranes. In individuals with cystic fibrosis, mutations prevent the proper functioning or production of this protein, leading to:
- Thick Mucus Production: The defective CFTR protein results in an imbalance of salt and water transport, causing mucus to become thick and sticky. This mucus builds up in various organs, particularly the lungs and pancreas.
- Respiratory Issues: The accumulation of mucus in the lungs obstructs airflow, creates an environment conducive to bacterial infections, and leads to chronic respiratory problems.
- Digestive Complications: In the pancreas, thick mucus blocks ducts that release digestive enzymes, resulting in malabsorption of nutrients and poor growth.
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