2.16 Cystic Fibrosis

• Genes can affect the phenotype of an organism
  • A gene codes for a single polypeptide
  • The polypeptide can affect the phenotype, e.g. it could form part of an enzyme or a membrane transport protein
• Genetic disorders are often caused by a mutation in a gene that results in a differently-functioning or non-functioning protein that alters the phenotype of the individual

Cystic fibrosis

• Cystic fibrosis is a genetic disorder of cell membranes caused by a recessive allele of the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene located on chromosome 7
  • This gene codes for the production of chloride ion channels required for secretion of sweat, mucus and digestive juices
  • A mutation in the CFTR gene leads to production of non-functional chloride channels
  • This reduces the movement of water by osmosis into the secretions
  • The result is that the body produces large amounts of thick, sticky mucus in the air passages, the digestive tract and the reproductive system
• Because cystic fibrosis is determined by a recessive allele, this means
  • People who are heterozygous won’t be affected by the disorder but are carriers
  • People must be homozygous recessive in order to have the disorder
  • If both parents are carriers the chance of them producing a child with cystic fibrosis is 1 in 4, or 25 %
  • If only one of the parents is a carrier with the other parent being homozygous dominant, there is no chance of producing a child with cystic fibrosis, as the recessive allele will always be masked by the dominant allele

Cystic fibrosis is a genetic disorder caused by a recessive allele

The respiratory system

• Mucus in the respiratory system is a necessary part of keeping the lungs healthy
  • It prevents infection by trapping microorganisms
  • This mucus is moved out of the respiratory tract by cilia 
• In people with cystic fibrosis, due to the faulty chloride ion channels, the cilia are unable to move as the mucus is so thick and sticky
• This means microorganisms are not efficiently removed from the lungs and lung infections occur more frequently
• Mucus builds up in the lungs and can block airways which limits gas exchange
  
  • The surface area for gas exchange is reduced which can cause breathing difficulties
• Physiotherapy can support people with cystic fibrosis to loosen the mucus in the airways and improve gas exchange

The digestive system

• Thick mucus in the digestive system can cause issues because
  • The tube to the pancreas can become blocked, preventing digestive enzymes from entering the small intestine
    • Digestion of some food may be reduced and therefore key nutrients may not be made available for absorption
  • The mucus can cause cysts to grow in the pancreas which inhibit the production of enzymes, further reducing digestion of key nutrients
  • The lining of the intestines is also coated in thick mucus, inhibiting the absorption of nutrients into the blood

The reproductive system

• Mucus is normally secreted in the reproductive system to prevent infection and regulate the progress of sperm through the reproductive tract after sexual intercourse
• The mucus in people with cystic fibrosis can cause issues in both men and women
  • In men the tubes of the testes can become blocked, preventing sperm from reaching the penis
  • In women thickened cervical mucus can prevent sperm reaching the oviduct to fertilise an egg